Do not DCHECK-fail when VDA has never populated.

Since it is the ctor that registers for OnWillDestroyStub, and not
Initialize(), it needs to be ok to call OnWillDestroyStub even if
Initialize() hasn't been called or has failed.

BUG=294442
TEST=Play video and run apprtc.appspot.com.

Review URL: https://codereview.chromium.org/24266008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225391 0039d316-1c4b-4281-b951-d872f2087c98
ChildProcessSecurityPolicy: Remove GrantCreateWriteFile.

The only user of GrantCreateWriteFile was the 'Save' file chooser. I figure if we're allowing the process to overwrite the file, it's harmless to allow it to also read the file.

This allows us to remove one permission from CPSP and simplify the code.

This is part of Step 2 of this refactoring plan:
https://docs.google.com/a/google.com/document/d/1QGkGWuwgSuaRqovz4wyb0upqPKDVsgYOFKt44E7gmOE/edit?usp=sharing

BUG=262142,263150

Review URL: https://codereview.chromium.org/24406003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225392 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225392