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The default app icon resource exists in the //extensions module resources
file and all implementations return the same icon. There's no need to delegate
this out.

BUG=none
TEST=app_shell, athena_main and chrome all run in debug without assertions
about missing resources

Review URL: https://codereview.chromium.org/586853004

Cr-Commit-Position: refs/heads/master@{#295916}