Rename inCompositingMode to staleInCompositingMode.

Any code that reads the compositingMode bit when the dirty
bit is set is getting a stale value. We should remove
these callers. This patch has no change in behavior.
Just a rename.

Review URL: https://codereview.chromium.org/244313008

git-svn-id: svn://svn.chromium.org/blink/trunk@172092 bbb929c8-8fbe-4397-9dbb-9b2b20218538