Add explicit API to clear backing store and canvas.

Once most resizes start happening asynchronously, the forcing of
size at the beginning of each test will become the odd man out.
This means that we can't hook into setWindowRect anymore and
instead need a special API, thereby pronounced didForceResize.

BUG=309760
R=eseidel, jochen, tony

Review URL: https://codereview.chromium.org/47953002

git-svn-id: svn://svn.chromium.org/blink/trunk@160732 bbb929c8-8fbe-4397-9dbb-9b2b20218538