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As discussed in https://codereview.chromium.org/1070433002/, this CL
allows arbitrary scale factors, which works because chromium uses the
resource most closely matching the scale factor (e.g. 1x for 1.25x,
but 2x for 1.9x) and scales it.

BUG=143619

Review URL: https://codereview.chromium.org/1078423002

Cr-Commit-Position: refs/heads/master@{#326851}