[iOS] Move KeyCommandsProvider to browser_view folder

This CL moves the KeyCommandsProvider class to the folder containing
BVC as it is mostly a helper class for BVC.

Bug: 944008
Change-Id: I2262044dabe3e73039dffa3597e4c1e64c1cfe7d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1530965
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#643091}
7 files changed