GitLab

Move the ChevronMenuButton class into its own file, and move the logic to show
the legacy (drop-down) extension action overflow menu from the
BrowserActionsContainer into there.

This is mainly because:
1. We'll eventually be getting rid of this legacy behavior, and this way, the
   removal is all concentrated in one area (ChevronMenuButton).
2. BrowserActionsContainer is already doing a ton (since it has both new
   overflow and normal logic). It doesn't need this, too.

As a bonus, refactored the lifetime of the overflow menu to be more
deterministic, and got rid of the back-and-forth between it and the
BrowserActionsContainer.

And all with a negative line count! Woohoo!

BUG=417085

Review URL: https://codereview.chromium.org/597783002

Cr-Commit-Position: refs/heads/master@{#297240}