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If OffscreenPreRaster is enabled, the viewport rect for tile priority
and the tile memory limit calculation will be based on the size of the
webview, instead of the size of visible area of the webview.

Design doc here
https://docs.google.com/document/d/1xOMjvbEIS3dMzpWXWrsl7qV8QOgGEIZL1QxgnfOXmq4/

BUG=460638

Review URL: https://codereview.chromium.org/944053004

Cr-Commit-Position: refs/heads/master@{#318126}