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/common/json_impl.cc

@@ -34,8 +34,17 @@ int GetJSONWriterOptions(cef_json_writer_options_t options) {
 CefRefPtr<CefValue> CefParseJSON(const CefString& json_string,
                                  cef_json_parser_options_t options) {
   const std::string& json = json_string.ToString();
-  base::Optional<base::Value> parse_result =
-      base::JSONReader::Read(json, GetJSONReaderOptions(options));
+  return CefParseJSON(json.data(), json.size(), options);
+}
+
+CefRefPtr<CefValue> CefParseJSON(const void* json,
+                                 size_t json_size,
+                                 cef_json_parser_options_t options) {
+  if (!json || json_size == 0)
+    return nullptr;
+  base::Optional<base::Value> parse_result = base::JSONReader::Read(
+      base::StringPiece(static_cast<const char*>(json), json_size),
+      GetJSONReaderOptions(options));
   if (parse_result) {
     return new CefValueImpl(
         base::Value::ToUniquePtrValue(std::move(parse_result.value()))