Add browser test for AEC dump.

Also:
- Code changes to avoid race between enable and disable dump. (This is extremely unlikely in practice due to the file picker dialog when enabling, but may happen in tests.)
- Allow enable dump after PeerConnectionFactory creation. This avoids race in tests and also has the benefit of letting a user enable the dump during an ongoing call.

R=jochen@chromium.org, phoglund@chromium.org

Review URL: https://codereview.chromium.org/101063003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242042 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242042