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There are two changes to make this work:
-Set the plugins width/height to 100%. We will soon be using this for the material layout as well.
-Allow MimeHandlerViewGuests to handle pinch gestures when they are full-page plugins.

BUG=453729

Review URL: https://codereview.chromium.org/904553002

Cr-Commit-Position: refs/heads/master@{#314742}