Fix for ~ContentCaptureSubscription() after BrowserThreads are torn down.

Fix for ~ContentCaptureSubscription() after BrowserThreads are torn down.

While debugging flaky browser_tests, it was revealed that a
WebContentsVideoCaptureMachine could actually outlive the tear-down of
the BrowserThreadImpl for the UI thread.  Not having been explicitly
stopped first, destructors in the object graph would execute clean up
code that would attempt operations that are invalid after the
BrowserThreadImpl for the UI thread has been torn down.

This change also includes a bunch of minor clean-ups.

BUG=396413
Committed: https://crrev.com/191dc4092d998a30ab39235a17f0f5f2d17dfb2b
Cr-Commit-Position: refs/heads/master@{#292513}

[miu, 2014-08-28 01:59:57]

miu@chromium.org changed reviewers:
+ hclam@chromium.org

[miu, 2014-08-28 01:59:57]

hclam: PTAL (and note my comments below).

https://codereview.chromium.org/514073002/diff/1/content/browser/media/captur...
File content/browser/media/capture/content_video_capture_device_core.h (left):

https://codereview.chromium.org/514073002/diff/1/content/browser/media/captur...
content/browser/media/capture/content_video_capture_device_core.h:148: //
threads (e.g., the IO BrowserThread), as well as code sharing.
Note: I removed references to the IO thread because it's no longer the IO thread
that calls into these objects (see VideoCaptureManager).

https://codereview.chromium.org/514073002/diff/1/content/browser/media/captur...
File content/browser/media/capture/web_contents_video_capture_device.cc (right):

https://codereview.chromium.org/514073002/diff/1/content/browser/media/captur...
content/browser/media/capture/web_contents_video_capture_device.cc:405: if
(!BrowserThread::IsMessageLoopValid(BrowserThread::UI))
This is the fix: Calling RenderViewHost::FromID(...) is invalid after the UI
BrowserThreadImpl has been torn down.

[Alpha Left Google, 2014-08-28 21:41:41]

LGTM.

[miu, 2014-08-28 22:55:51]

The CQ bit was checked by miu@chromium.org

[commit-bot: I haz the power, 2014-08-28 22:57:03]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/miu@chromium.org/514073002/1

[commit-bot: I haz the power, 2014-08-29 00:08:37]

Committed patchset #1 (id:1) as 2ef42592956228c0da0d9514db51e67a16ef73c7

[commit-bot: I haz the power, 2014-09-10 03:04:04]

Patchset 1 (id:??) landed as
https://crrev.com/191dc4092d998a30ab39235a17f0f5f2d17dfb2b
Cr-Commit-Position: refs/heads/master@{#292513}