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This CL introduces a method PDFiumPage::PopulateButton() which reads
button form fields (radio button, checkbox and push button) from the PDF
document and stores relevant information in a vector within PDFiumPage.

The CL also includes a new test file with sample radio buttons, checkbox
and push button. A unit test is also added to validate the new method.

Bug: 1030242
Change-Id: I1f5102f8c20027abe785fa1833c2ecc9cbce8734
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2249285
Commit-Queue: Mansi Awasthi <maawas@microsoft.com>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Ankit Kumar 🌪️ <ankk@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#784392}