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.

libcef_dll/cpptoc/request_handler_cpptoc.cc

@@ -9,7 +9,7 @@
 // implementations. See the translator.README.txt file in the tools directory
 // for more information.
 //
-// $hash=68840d18a25efe4d749e480225d7ac3caacd4723$
+// $hash=716501855da0203723eb18bfb9e518c7b155f110$
 //
 
 #include "libcef_dll/cpptoc/request_handler_cpptoc.h"
@@ -396,6 +396,26 @@ void CEF_CALLBACK request_handler_on_render_process_terminated(
       CefBrowserCToCpp::Wrap(browser), status);
 }
 
+void CEF_CALLBACK request_handler_on_document_available_in_main_frame(
+    struct _cef_request_handler_t* self,
+    cef_browser_t* browser) {
+  shutdown_checker::AssertNotShutdown();
+
+  // AUTO-GENERATED CONTENT - DELETE THIS COMMENT BEFORE MODIFYING
+
+  DCHECK(self);
+  if (!self)
+    return;
+  // Verify param: browser; type: refptr_diff
+  DCHECK(browser);
+  if (!browser)
+    return;
+
+  // Execute
+  CefRequestHandlerCppToC::Get(self)->OnDocumentAvailableInMainFrame(
+      CefBrowserCToCpp::Wrap(browser));
+}
+
 }  // namespace
 
 // CONSTRUCTOR - Do not edit by hand.
@@ -414,6 +434,8 @@ CefRequestHandlerCppToC::CefRequestHandlerCppToC() {
   GetStruct()->on_render_view_ready = request_handler_on_render_view_ready;
   GetStruct()->on_render_process_terminated =
       request_handler_on_render_process_terminated;
+  GetStruct()->on_document_available_in_main_frame =
+      request_handler_on_document_available_in_main_frame;
 }
 
 // DESTRUCTOR - Do not edit by hand.