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The previous logic doesn't account for divide truncation and
border texels, resulting in an additional visible tile with
only a couple of rows visible. This is hidden by the URL bar
sometimes, but it still affects full-screen, web-apps and
desktop browsers.

BUG=365877

Review URL: https://codereview.chromium.org/626113004

Cr-Commit-Position: refs/heads/master@{#299201}