Implement detaching the TabModelSelector from the CompositorViewHolder

Implement detaching the TabModelSelector from the CompositorViewHolder

Essentially returns the CompositorViewHolder to a state where the TabModelSelector hasn't been set, and allows the TabModelSelector to be re-attached.
This allows VR Shell to control the size of CVCs, as the CompositorViewHolder no longer owns the CVC while in VR.

BUG=680240
Review-Url: https://codereview.chromium.org/2641043002
Cr-Commit-Position: refs/heads/master@{#445419}
(cherry picked from commit 19d9d40d78985410d311f7937186943928580fb1)

Review-Url: https://codereview.chromium.org/2658903002 .
Cr-Commit-Position: refs/branch-heads/2987@{#118}
Cr-Branched-From: ad51088c0e8776e8dcd963dbe752c4035ba6dab6-refs/heads/master@{#444943}
Committed: https://chromium.googlesource.com/chromium/src/+/9341b1c39c879f65c46421c7876c310d590a8de0

[mthiesse, 2017-01-26 20:56:18]

Description was changed from

==========
Implement detaching the TabModelSelector from the CompositorViewHolder

Essentially returns the CompositorViewHolder to a state where the
TabModelSelector hasn't been set, and allows the TabModelSelector to be
re-attached.
This allows VR Shell to control the size of CVCs, as the CompositorViewHolder no
longer owns the CVC while in VR.

BUG=680240

Review-Url: https://codereview.chromium.org/2641043002
Cr-Commit-Position: refs/heads/master@{#445419}
(cherry picked from commit 19d9d40d78985410d311f7937186943928580fb1)
==========

to

==========
Implement detaching the TabModelSelector from the CompositorViewHolder

Essentially returns the CompositorViewHolder to a state where the
TabModelSelector hasn't been set, and allows the TabModelSelector to be
re-attached.
This allows VR Shell to control the size of CVCs, as the CompositorViewHolder no
longer owns the CVC while in VR.

BUG=680240

Review-Url: https://codereview.chromium.org/2641043002
Cr-Commit-Position: refs/heads/master@{#445419}
(cherry picked from commit 19d9d40d78985410d311f7937186943928580fb1)

Review-Url: https://codereview.chromium.org/2658903002 .
Cr-Commit-Position: refs/branch-heads/2987@{#118}
Cr-Branched-From:
ad51088c0e8776e8dcd963dbe752c4035ba6dab6-refs/heads/master@{#444943}
Committed:
https://chromium.googlesource.com/chromium/src/+/9341b1c39c879f65c46421c7876c...
==========

[mthiesse, 2017-01-26 20:56:19]

Committed patchset #1 (id:1) manually as
9341b1c39c879f65c46421c7876c310d590a8de0.