[refactor] Split up some large switch statements.

BUG=245025
R=rsleevi@chromium.org

Review URL: https://codereview.chromium.org/147533002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248108 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248108
Remove ui::Compositor::ReadPixels

All callers now use asynchronous CopyOutputRequest on a layer and so
this function can be removed entirely.

PendingSwap could probably be simplified more as there is only one type
in the short term, but PendingSwap will go away entirely once
ui::Compositor becomes a SchedulerClient and so it's left as-is.

BUG=332167

Review URL: https://codereview.chromium.org/127143002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248109 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248109