GitLab

This type is specific to NavButtonProvider, which only exists
for Linux, so the type is moved inside NavButtonProvider

R=sky
BUG=None

Change-Id: I910bd42a4ead9a8b6add268f021d01dbc6eb41b5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2092877
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748781}