Disable ExtensionApiTest.TabMuted as it's flaky.
Disable ExtensionApiTest.TabMuted as it's flaky.

BUG=521410
TBR=miu@chromium.org

Review URL: https://codereview.chromium.org/1291623003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#343612}

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