Add initial chrome runtime support (see issue #2969)

Running `cefsimple --enable-chrome-runtime` will create and run a
Chrome browser window using the CEF app methods, and call
CefApp::OnContextInitialized as expected. CEF task methods also
work as expected in the main process. No browser-related methods or
callbacks are currently supported for the Chrome window, and the
application will exit when the last Chrome window closes.

The Chrome runtime requires resources.pak, chrome_100_percent.pak
and chrome_200_percent.pak files which were not previously built
with CEF. It shares the existing locales pak files which have been
updated to include additional Chrome-specific strings.

On Linux, the Chrome runtime requires GTK so use_gtk=true must be
specified via GN_DEFINES when building.

This change also refactors the CEF runtime, which can be tested in
the various supported modes by running:
$ cefclient
$ cefclient --multi-threaded-message-loop
$ cefclient --external-message-pump

tools/gn_args.py

@@ -232,6 +232,10 @@ def GetRecommendedDefaultArgs():
     # TODO(cef): Remove this flag once we require a newer host system.
     result['fatal_linker_warnings'] = False
 
+    # GTK is enabled by default for the Chrome runtime (see issue #2969).
+    # Disable GTK when building the CEF runtime only (see issue #2014).
+    result['use_gtk'] = False
+
   return result
 
 
@@ -266,9 +270,6 @@ def GetRequiredArgs():
     # can't be enforced by assert().
     result['enable_linux_installer'] = False
 
-    # Build without GTK dependencies (see issue #2014).
-    result['use_gtk'] = False
-
   if platform == 'macosx':
     # Always generate dSYM files. The make_distrib script will fail if
     # enable_dsyms=true is not explicitly set when is_official_build=false.