Fix prerenders firing external protocol prompts.

This regressed in https://codereview.chromium.org/124393002 due to a race. It
is possible for a PrerenderContents to be a doomed, but not yet destroyed,
state. Handle this state accordingly.

- If LaunchURL in ChromeRDHDelegate races with tab close and can't find a
  WebContents, don't launch the external protocol.
- PrerenderManager::GetPrerenderContents no longer returns NULL for prerenders
  in the to_delete_prerenders_ list.
- PrerenderBrowserTest.*UnsupportedScheme* no longer tries to navigate to the
  destination URL.
- Re-enable PrerenderCancelSubresourceUnsupportedScheme in hopes this or other
  changes alleviate the flakiness. Otherwise it can be disabled again.
- Assert that external protocol handlers do not run in prerender browsertests,
  so this regression is caught next time.

BUG=333355,249179
TEST=PrerenderBrowserTest.*UnsupportedScheme*

Review URL: https://codereview.chromium.org/134753002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244729 0039d316-1c4b-4281-b951-d872f2087c98
[PPAPI] Add media stream audio track c++ API

BUG=330851

Review URL: https://codereview.chromium.org/133583006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244730 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 244729
[PPAPI] Pepper MediaStream API audio thunk implementation

BUG=330851

Review URL: https://codereview.chromium.org/133703004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244731 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 244731
Roll breakpad DEPS to r1271.

r1271: Add -[BreakpadController setParametersToAddAtUploadTime:] for iOS.

BUG=326292

Review URL: https://codereview.chromium.org/133703007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244732 0039d316-1c4b-4281-b951-d872f2087c98
Update .DEPS.git

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244733 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 244733