PDF Viewer: Show PDF page size among properties

The page size string is localized in the plugin.

This is the last property that needed to be implemented, so enable
the document properties feature flag to enabled by default.

Bug: 1184341
Change-Id: I7f98c4e82a3b90b6a870fa06fe2199bc2e4c84a4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2713828


Commit-Queue: Daniel Hosseinian <dhoss@chromium.org>
Reviewed-by: dpapad <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#859598}