[PPAPI] Fixed FileSystems from JavaScript not having a context.

Removed the "open existing file system" overload of
PepperFileSystemBrowserHost constructor, replacing it with a method
OpenExisting that asynchronously opens the file system, correctly
obtaining a FileSystemContext.

Rewrote PepperRendererConnection::OnMsgCreateResourceHostsFromHost to
deal with having to call an asynchronous function before sending the
reply to the plugin.

Re-enabled disabled test that was broken due to this bug and added
an additional regression test.

BUG=314007

Review URL: https://codereview.chromium.org/55133010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232605 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232605