Gallery: do not set zoom more than once in a frame.
Gallery: do not set zoom more than once in a frame.

BUG=591033
TEST=manually tested as described in the issue.

Review URL: https://codereview.chromium.org/1770903003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#379774}

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