Add support for direct DevTools protocol messaging (fixes issue #2961).

This change allows the client to directly send and receive DevTools
protocol messages (send method calls, and receive method results and
events) without requiring a DevTools front-end or remote-debugging
session.

This change includes additional supporting changes:
- Add a new CefRequestHandler::OnDocumentAvailableInMainFrame
  callback (see issue #1454).
- Add a CefParseJSON variant that accepts a UTF8-encoded buffer.
- Add a `--devtools-protocol-log-file=<path>` command-line flag for
  logging protocol messages sent to/from the DevTools front-end
  while it is displayed. This is useful for understanding existing
  DevTools protocol usage.
- Add a new "libcef_static_unittests" executable target to support
  light-weight unit tests of libcef_static internals (e.g. without
  requiring exposure via the CEF API). Files to be unittested are
  placed in the new "libcef_static_unittested" source_set which is
  then included by both the existing libcef_static library and the
  new unittests executable target.
- Linux: Remove use_bundled_fontconfig=false, which is no longer
  required and causes unittest build errors (see issue #2424).

This change also adds a cefclient demo for configuring offline mode
using the DevTools protocol (fixes issue #245). This is controlled
by the "Offline mode" context menu option and the `--offline`
command-line switch which will launch cefclient in offline mode. When
cefclient is offline all network requests will fail with
ERR_INTERNET_DISCONNECTED and navigator.onLine will return false when
called from JavaScript in any frame. This mode is per-browser so
newly created browser windows will have the default mode. Note that
configuring offline mode in this way will not update the Network tab
UI ("Throtting" option) in a displayed DevTools front-end instance.

cef_paths.gypi

@@ -8,7 +8,7 @@
 # by hand. See the translator.README.txt file in the tools directory for
 # more information.
 #
-# $hash=21f0ab1e9902e4a47bf2893a4a383d33bd8161e2$
+# $hash=30eed8c81da55c640eb6a491283d1c00fb59d635$
 #
 
 {
@@ -26,6 +26,7 @@
       'include/cef_context_menu_handler.h',
       'include/cef_cookie.h',
       'include/cef_crash_util.h',
+      'include/cef_devtools_message_observer.h',
       'include/cef_dialog_handler.h',
       'include/cef_display_handler.h',
       'include/cef_dom.h',
@@ -122,6 +123,7 @@
       'include/capi/cef_context_menu_handler_capi.h',
       'include/capi/cef_cookie_capi.h',
       'include/capi/cef_crash_util_capi.h',
+      'include/capi/cef_devtools_message_observer_capi.h',
       'include/capi/cef_dialog_handler_capi.h',
       'include/capi/cef_display_handler_capi.h',
       'include/capi/cef_dom_capi.h',
@@ -260,6 +262,8 @@
       'libcef_dll/ctocpp/domvisitor_ctocpp.h',
       'libcef_dll/ctocpp/delete_cookies_callback_ctocpp.cc',
       'libcef_dll/ctocpp/delete_cookies_callback_ctocpp.h',
+      'libcef_dll/ctocpp/dev_tools_message_observer_ctocpp.cc',
+      'libcef_dll/ctocpp/dev_tools_message_observer_ctocpp.h',
       'libcef_dll/ctocpp/dialog_handler_ctocpp.cc',
       'libcef_dll/ctocpp/dialog_handler_ctocpp.h',
       'libcef_dll/cpptoc/dictionary_value_cpptoc.cc',
@@ -566,6 +570,8 @@
       'libcef_dll/cpptoc/domvisitor_cpptoc.h',
       'libcef_dll/cpptoc/delete_cookies_callback_cpptoc.cc',
       'libcef_dll/cpptoc/delete_cookies_callback_cpptoc.h',
+      'libcef_dll/cpptoc/dev_tools_message_observer_cpptoc.cc',
+      'libcef_dll/cpptoc/dev_tools_message_observer_cpptoc.h',
       'libcef_dll/cpptoc/dialog_handler_cpptoc.cc',
       'libcef_dll/cpptoc/dialog_handler_cpptoc.h',
       'libcef_dll/ctocpp/dictionary_value_ctocpp.cc',