arc: Fix IconLoader test flakiness.
arc: Fix IconLoader test flakiness.

BUG=601157
TEST=Multiple runs of IconLoader test showed no test failure.

Review URL: https://codereview.chromium.org/1863263002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#385586}

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