Fixed the issue that Ctrl+C can copy non-copyable text from PDF

This CL adds a check for whether the plugin allows copy operations in
WebPluginContainerImpl::Copy() so that "Ctrl+C" will no longer copy the
texts from a PDF without the copy permission.

Bug: 1380985
Change-Id: Ie84543616bfd9cc277595b8421ec86335a8d85b2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4034975


Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Nigi <nigi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1076492}