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This fixes a regression I introduced after [1] which removed colormap
creation code.  It turns out the colormap is still necessary for
transparent visuals, so this CL adds it back.

[1] https://chromium.googlesource.com/chromium/src/+/ae0a09cefaca3445bf43b0abefe3d068794388c0

R=sky

Change-Id: I3d29ced609ab3f915f9b7cdbdce6f4e8b2a5dce1
Bug: 1106334,1107385
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2311070
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Auto-Submit: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#790611}