Remove WebWidget::didExitCompositingMode.

After https://codereview.chromium.org/256573003/ this is no longer
called.

R=abarth
BUG=366130

Review URL: https://codereview.chromium.org/259633002

git-svn-id: svn://svn.chromium.org/blink/trunk@172654 bbb929c8-8fbe-4397-9dbb-9b2b20218538