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A recent CL introduced some new GN dependencies that broke internal Cast
variants:
  https://codereview.chromium.org/2354633002

Correct the dependencies here. Also put stronger asserts in x11 targets.

BUG=

Review-Url: https://codereview.chromium.org/2371583002
Cr-Commit-Position: refs/heads/master@{#422860}