[PDF Ink Signatures] Get and observe preferences in PdfViewWebPlugin

blink::RendererPreferences contains an entry for whether caret browsing
mode should be enabled. Use this entry for the PDF viewer by doing the
following:

1. Add OnRendererPreferencesUpdated() to WebViewObserver to notify
   observers of renderer preference updates.
2. Make PdfViewWebPlugin a WebViewObserver.
3. Use the renderer preferences to set caret browsing mode in
   PDFiumEngine after the PDF loads and in future preference updates.

Other renderer preferences will be needed by the PDF viewer in the
future, such as the caret blink interval, and
OnRendererPreferencesUpdated() will already be supplying the
preferences.

Bug: 427242881, 427778119
Change-Id: I945a69dc1448780c048db17099a1507148a7b539
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6788796
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Andy Phan <andyphan@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1493620}