[Android WebView] Add DevTools Timeline instrumentation for embedder callbacks

This allows displaying time spent in WebView callbacks code, which can
affect loading and page displaying times.

BUG=344871

Review URL: https://codereview.chromium.org/171793006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252919 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252919
Update shell_aura.cc to get aura::Window for dispatcher.

This change is related to CL: https://codereview.chromium.org/172743002 where
RootWindow was renamed to WindowEventDispatcher.

This fixes the Ozone build which uses shell_aura.cc

TBR=ben@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/177713003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252920 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252920