Don't disable forceCompositingMode in PinchViewportTest.TestWebViewResizedBeforeAttachment.

We're getting rid of the forceCompositingMode setting. This
is one of two remaining callers that sets it to false. For now,
disable accelerated compositing entirely. Verified that the
test still fails when r173949 is reverted.

BUG=377851,363772

Review URL: https://codereview.chromium.org/306453006

git-svn-id: svn://svn.chromium.org/blink/trunk@174914 bbb929c8-8fbe-4397-9dbb-9b2b20218538