Better expose PDFiumEngine::plugin_size() callers that fail DCHECK

Use DUMP_WILL_BE_NOTREACHED() in PDFiumEngine::plugin_size() instead of
a DCHECK(), so that the DCHECK() failures consistently generate
non-fatal dumped in all builds. Temporarily un-inline plugin_size() to
avoid putting more code into the header.

Bug: 40193305
Change-Id: I92459e6d36fe33d296404486dd4b93615113a719
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5618930


Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Alan Screen <awscreen@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1313170}