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/wrapper/libcef_dll_wrapper.cc

@@ -9,7 +9,7 @@
 // implementations. See the translator.README.txt file in the tools directory
 // for more information.
 //
-// $hash=c565ea9030eeed15f24fe420b828453a23dbc6ba$
+// $hash=7db8dbe24a2510d9ae0649f1569909711017c064$
 //
 
 #include "include/capi/cef_app_capi.h"
@@ -557,6 +557,24 @@ CEF_GLOBAL CefRefPtr<CefValue> CefParseJSON(const CefString& json_string,
   return CefValueCToCpp::Wrap(_retval);
 }
 
+NO_SANITIZE("cfi-icall")
+CEF_GLOBAL CefRefPtr<CefValue> CefParseJSON(const void* json,
+                                            size_t json_size,
+                                            cef_json_parser_options_t options) {
+  // AUTO-GENERATED CONTENT - DELETE THIS COMMENT BEFORE MODIFYING
+
+  // Verify param: json; type: simple_byaddr
+  DCHECK(json);
+  if (!json)
+    return nullptr;
+
+  // Execute
+  cef_value_t* _retval = cef_parse_json_buffer(json, json_size, options);
+
+  // Return type: refptr_same
+  return CefValueCToCpp::Wrap(_retval);
+}
+
 NO_SANITIZE("cfi-icall")
 CEF_GLOBAL CefRefPtr<CefValue> CefParseJSONAndReturnError(
     const CefString& json_string,