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- Makes components_browsertests swarmable.
- Adds components_browsertests to the main waterfall for linux, mac and
  win bots.
- Updates the chromium tryserver to run components_browsertests as
  swarmable.
- Add content_shell as a dependency for components_browsertests for
  Mac, which is needed for the Content Shell helper on that platform.

The original CL did not add content_shell as a dependency for
components_browsertests, which made the build step for the isolate flaky
dependent upon whether content_shell happened to have already been built.

BUG=311270

Original CL:
Review: https://codereview.chromium.org/863023003/
Committed: https://crrev.com/7a3df3b7171fa87f556d22803fdc02a67e975865
Cr-Commit-Position: refs/heads/master@{#312886}

Revert CL:
https://codereview.chromium.org/856323003/
Committed: https://crrev.com/27ac8082963904942aaebcbf85620ae6e27ad5c9
Cr-Commit-Position: refs/heads/master@{#312976}

Review URL: https://codereview.chromium.org/874843002

Cr-Commit-Position: refs/heads/master@{#313163}