Move PinchViewport to the Oilpan heap.

By having it on the heap, the FrameHost back reference can be properly
accounted for during GCs.

R=haraken
BUG=340522

Review URL: https://codereview.chromium.org/642003002

git-svn-id: svn://svn.chromium.org/blink/trunk@183532 bbb929c8-8fbe-4397-9dbb-9b2b20218538