Add an alternate paintContents method to WebContentLayerClient.

The new method does not take an opaque rect out-parameter. And the old
methods all just call the new method and discard the resulting opaque
rect.

Chromium side is not using this opaque rect anymore and we're removing
it there, so making the matching change to blink. This removes a call
to GraphicsContext::setRegionTrackingMode(), but there are others.

R=fmalita@chromium.org, jamesr@chromium.org, schenney@chromium.org
BUG=413479

Review URL: https://codereview.chromium.org/554183004

git-svn-id: svn://svn.chromium.org/blink/trunk@181996 bbb929c8-8fbe-4397-9dbb-9b2b20218538