Create MimeHandlerViewGuest in the same process as the extension handling the mime...

Create MimeHandlerViewGuest in the same process as the extension handling the mime type.

This will make sure we do not keep lingering storage stuff like before, as
previously we were creating different site instances for each type of mime
type.

BUG=415358
Test=No visible change is expected.
To the test process behavior, you'd need --out-of-process-pdf and
--enable-mime-handler-view flag and apply the patch to enable oop PDF:
http://crrev.com/392813003/.
Now if you navigate to two PDF files in the tab and check chrome's
task manager, you'd see the PDF extension and the mime-handler-views
ended up in the same process.

Review URL: https://codereview.chromium.org/580963003

Cr-Commit-Position: refs/heads/master@{#295394}