A Surface may hold both an active and a pending CompositorFrame at once.
Surfaces referenced by either CompositorFrame must be retained in order
to ensure correctness once the pending frame is activated. This CL
tracks references from both pending and active CompositorFrames within
a surface.

This CL also fixes a tear down issue in SurfaceDependencyTracker where Surfaces
could continue to call back into SurfaceDependencyTracker after it was
destroyed. SurfaceDependencyTracker stops observing Surfaces in its destructor.

BUG=672062
TBR=jam@chromium.org for mechanical renaming in content/
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel

Review-Url: https://codereview.chromium.org/2688043002
Cr-Commit-Position: refs/heads/master@{#449543}