GitLab

This CL includes adding:
- Feature flag for gating Projector feature.
- ProjectorController: the main controller
- ProjectorUiController: the controller responsible for interacting
with the UI.
- ProjectorMetadataController: the controller responsible for recording
and saving the screencast metadata.
- ProjectorMetadata: model of the projector metadata.

Follow up CLs will add more implementations and unit tests. More detail
about Projector at: go/projector-prd

Bug: 1164029
Test: unit tests
Change-Id: I808a044c444a999ecfdbcf1615949cdd4df182b6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2627441
Commit-Queue: Li Lin <llin@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Yilkal Abe <yilkal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845315}