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This CL adds ability to tab across links and highlights (along with
already supported widgets) behind a newly added feature flag
kTabAcrossPDFAnnotations. This is done by calling the newly added
PDFium API FPDFAnnot_SetFocusableSubtypes().
A unit test has also been added to validate the added support.
Here is the related CL:
https://pdfium-review.googlesource.com/c/pdfium/+/60011/

Bug: 994500
Change-Id: I00312c00f3e6373949a4723cc65778fdb32ab963
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1783695Reviewed-by: Ian Prest <iapres@microsoft.com>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Neha Gupta <negupta@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#759380}