Initial cameo launcher based on chromium content API

This commit is based on chromium@196638 revision. It implements a basic
cameo launch which is able to able to load a URL and render the page in
a native window.

Highlights:
o Support both Linux and Windows.
o Abstract the HTML page execution environment to Runtime class.
o Abstract the native window impl to a NativeAppWindow interface.
o Add testing framework and several test cases to verify the launcher
  can work as expected.

Build and run cameo launcher:
o Make sure the chromium codebase is synced to 196638 revision using
  gclient sync and the patch for cameo resource ID is applied to it
o Run 'python cameo/gyp_cameo" to generate building files
o Use ninja to build it via 'ninja -C out/Release cameo'
o Launch 'out/Release/cameo <URL>'

Run and debug a test case:
o Run all browser tests:
  > out/Release/cameo_browsertest
o Debug a test case on Linux:
  > gdb --args out/Release/cameo_browsertest \
    --gtest_filter=CameoRuntimeTest.LoadURLAndClose \
    --single_process
o Debug a test case on Windows:
  > out/Release/cameo_browsertest \
    --gtest_filter=CameoRuntimeTest.LoadURLAndClose \
    --single_process \
    --wait-for-debugger (or --renderer-startup-dialog for renderer process)
  A MessageBox will be popup with showing browser process ID. You could attach
  it to VS debugger.

Known issues:
o Cannot work with http proxy on Linux. As a result, it will fail to
  load a URL if using http proxy on Linux.