This CL updates the Link structure to include the underlying text range.
The underlying text range is being populated using the newly added
FPDFLink_GetTextRange() API. This would enable us to position links along
with text in the accessibility tree.

Bug: 981448
Change-Id: I4551f70c21e2846f9a85c19c982f17dc52e0b25c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1710275
Commit-Queue: Virender Singh <virens@microsoft.com>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Kevin Babbitt <kbabbitt@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#686341}