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This patch ensures that we can the raster scale to min contents scale.
We already did this, but later could modify the value again to be lower
than MinContentsScale. Now, the clamp happens near the end of the
function.

R=danakj
BUG=429397

Review URL: https://codereview.chromium.org/699053003

Cr-Commit-Position: refs/heads/master@{#302636}