Change VideoFrame close() to destroy()

This CL changes the semantics of releasing the resources held by a
blink::VideoFrame, after one is done with the frame. Previously, every
frame created had to be explicitly closed or GC'ed before the underlying
media::VideoFrame was released.

This CL adds a blink::VideoFrame::Handle which can be shared across
frames. Destroying one frame destroys all frames sharing the same
handle. This makes it easier to handle serialization/desirialization
cases, during which there can be dangling reference held by intermediary
objects that are impossible to close.

blink::VideoFrames constructed from a media::VideoFrame reference will
create a new Handle, whereas those constructed from a Handle will
share it.

Bug: 1096715
Change-Id: I775abe4e697d01ac14d98af81c78dbc3fd6bd5cb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2343846
Commit-Queue: Thomas Guilbert <tguilbert@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#797005}