[unseasoned-pdf] Time out PDFExtensionClipboardTest gracefully

Times out more gracefully from DoActionAndCheckClipboard() by cleaning
up and returning early, instead of triggering DCHECKs.

The DCHECKs occur because base::RunLoop::Run() returns after a timeout,
and so tests continue running for a short time before being terminated
by the test runner. This leads to multiple DoActionAndCheckClipboard()
calls while in an unexpected state.

Bug: 1229631
Change-Id: I3dc85426428d6c0b1d0c966e3c470b0c8d127347
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3071907


Auto-Submit: K. Moon <kmoon@chromium.org>
Commit-Queue: Daniel Hosseinian <dhoss@chromium.org>
Reviewed-by: Daniel Hosseinian <dhoss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#908709}