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Implements several new checks in the plugin, gated behind a flag:
- Only one of {virtual,override,final} should be ever used.
- Destructors must also be annotated correctly.
- A virtual final method that doesn't override anything should be
  devirtualized.

The test harness for the Chrome plugin has also been updated to stop
it from littering the source tree with object files, and to make the
golden files easier to update.

BUG=417463

Review URL: https://codereview.chromium.org/597863002

Cr-Commit-Position: refs/heads/master@{#297129}