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

tests/cefsimple/cefsimple_linux.cc

@@ -9,6 +9,7 @@
 #endif
 
 #include "include/base/cef_logging.h"
+#include "include/cef_command_line.h"
 
 #if defined(CEF_X11)
 namespace {
@@ -52,9 +53,18 @@ int main(int argc, char* argv[]) {
   XSetIOErrorHandler(XIOErrorHandlerImpl);
 #endif
 
+  // Parse command-line arguments for use in this method.
+  CefRefPtr<CefCommandLine> command_line = CefCommandLine::CreateCommandLine();
+  command_line->InitFromArgv(argc, argv);
+
   // Specify CEF global settings here.
   CefSettings settings;
 
+  if (command_line->HasSwitch("enable-chrome-runtime")) {
+    // Enable experimental Chrome runtime. See issue #2969 for details.
+    settings.chrome_runtime = true;
+  }
+
 // When generating projects with CMake the CEF_USE_SANDBOX value will be defined
 // automatically. Pass -DUSE_SANDBOX=OFF to the CMake command-line to disable
 // use of the sandbox.