[Web, Keyboard] Handle unreasonable modifier flags on ShiftLeft+MetaLeft

[dkwingsmt]

Fixes flutter/flutter#99297.

In extremely rare cases, the modifier flags of an event will contradict to the event itself. This occurs on Chrome on Linux while holding ShiftLeft, where the MetaLeft down event has metaKey true, while the Meta up event has metaKey false.

This PR fixes it by ignoring the key of the current event during synthesization. After all, the current event is a ground truth.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See testing the engine for instructions on
  writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

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[gspencergoog]

You might consider filing an issue with the Chrome team about this: it seems like a bug on their part.