[unseasoned-pdf] Add some text editing methods in PdfViewWebPlugin.

Add HasEditableText(), CanUndo() CanRedo() in PdfViewWebPlugin.

While at it, mark the methods with the same names in PDFEngine const.

Bug: 1204729
Change-Id: I63bca29cdcd051194a252f9b5248056bca23bc45
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2874983


Commit-Queue: Hui Yingst <nigi@chromium.org>
Reviewed-by: Daniel Hosseinian <dhoss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#881068}