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Occlusion tracker converts Rects to QuadF, then does MapClippedQuad,
then BoundingBox(), then ToEnclosedRect(). Instead, add methods to
MathUtil that mirror the {Map,Project}EnclosingClippedRect
methods but for an enclosed rect instead, that work only when the
transform preserves 2d axis alignment.

In the common case, this avoids converting the integers to floats just
to convert back to ints using the costly safe conversion methods.

R=enne

Review URL: https://codereview.chromium.org/495873002

Cr-Commit-Position: refs/heads/master@{#291785}