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This is a reland of 849315dd

Original change's description:
> Make AppMenuModel keep most submenus in a vector.
> 
> They are only needed for ownership, so no need to give them names and concrete
> types. Removes some forward declarations from the header and serendipitously
> happens to remove GOOGLE_CHROME_BUILD from the header as well.
> 
> No intended behavior change.
> 
> Bug: 961769
> Change-Id: Ibaaf3375bd9d367b4cc83b9a8ec3e64193750a69
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1894032
> Commit-Queue: Robert Sesek <rsesek@chromium.org>
> Auto-Submit: Nico Weber <thakis@chromium.org>
> Reviewed-by: Robert Sesek <rsesek@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#711431}

Bug: 961769
Change-Id: Ic076b82d89cdcb4dcf97e88a4e04b88e22e7e504
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1893003
Commit-Queue: Nico Weber <thakis@chromium.org>
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Auto-Submit: Nico Weber <thakis@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711512}