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We use View.getGlobalVisibleRect, and the transform and viewport from
functor to compute info for the viewport:
* the tile memory budget, which ideally is computed from the size of
  the visible portion of webview
* the viewport and transform used to raster the right tiles at the
  right scale

We usually use getGlobalVisibleRect and transform from functor
together if is_layer is false, making the assumption that they both
transform between the window space and view space. This turns out not
to be true.

Instead, use getGlobalVisibleRect to compute the tile budget. And
always use the viewport and transform from functor together for
tile raster, since they are guaranteed to work together.

Change-Id: I4d1346501dab14dbb7b7ca160e0c88fee64aa3ca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1506759
Auto-Submit: Bo <boliu@chromium.org>
Reviewed-by: Eric Karl <ericrk@chromium.org>
Reviewed-by: Tobias Sargeant <tobiasjs@chromium.org>
Commit-Queue: Bo <boliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638647}