Implement PdfAXActionTarget::SetSelection method

Added a new action PP_PDF_SET_SELECTION to PP_PdfAccessibilityAction and
selection specific data in PP_PdfAccessibilityActionData. Implemented
PdfAXActionTarget::SetSelection method to send selection data to plugin.
PDFiumEngine handles PP_PDF_SET_SELECTION action and set the selection
in plugin. Added the tests to validate if write action data is sent from
Mimehandler to Plugin for selection and also if the action is correctly
executed in Plugin.

Bug: 1023283
Change-Id: I9112698757d7516d38f29e88174f10da71673b1c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1934127
Commit-Queue: Mansi Awasthi <maawas@microsoft.com>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Kevin Babbitt <kbabbitt@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#727044}

• components/pdf/renderer/pdf_accessibility_tree.cc[diff]
• components/pdf/renderer/pdf_accessibility_tree.h[diff]
• components/pdf/renderer/pdf_accessibility_tree_browsertest.cc[diff]
• components/pdf/renderer/pdf_ax_action_target.cc[diff]
• components/pdf/renderer/pdf_ax_action_target.h[diff]
• pdf/pdfium/accessibility_unittest.cc[diff]
• pdf/pdfium/pdfium_engine.cc[diff]
• pdf/pdfium/pdfium_engine.h[diff]
• pdf/pdfium/pdfium_page.cc[diff]
• pdf/pdfium/pdfium_page.h[diff]
• ppapi/c/private/ppp_pdf.h[diff]
• ppapi/proxy/ppapi_messages.h[diff]