cc: Allow sending BeginMainFrame before draw or activation

BeginMainFrame before draw:
Enabled by default because of concurrency benefits.
When main thread painting, this will block the main thread from
finishing the commit until the active tree has been drawn.
When impl side painting, this does not block the main thread.

The following flag will disable sending BeginMainFrame to the
main thread before we draw the previous commit:
--disable-main-frame-before-draw

BeginMainFrame before activation:
Disabled by default because of latency concerns.
This applies only when impl-side-painting is enabled and will
cause the main thread to block finishing the commit until the
pending tree is activated.

The following flags control whether BeginMainFrame is sent to
the main thread before or after the previous commit has
activated:
--enable-main-frame-before-activation
--disable-main-frame-before-activation

BUG=293941

Review URL: https://codereview.chromium.org/23907006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256454 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256454