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This patch ensures that the skewport contains the visible rect. This
is a required that could be violated by extreme visible rects that
would overflow integer math. The fix is to add an extra union.
Includes a test.

R=enne
BUG=487130

Review URL: https://codereview.chromium.org/1131383007

Cr-Commit-Position: refs/heads/master@{#330600}