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This change removes the pp::TouchInputEvent dependency from
PDFiumEngine. There are two reasons to remove this dependency
- Ability to unit test touch event handling.
- Facilitate the PPAPI deprecation by moving some PPAPI dependencies
  out of PDFiumEngine.

Bug: 1091834
Change-Id: I500faa950bbc8459e78f66fbe84db63f505777c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2340360
Commit-Queue: Virender Singh <virens@microsoft.com>
Reviewed-by: Ankit Kumar 🌪️ <ankk@microsoft.com>
Reviewed-by: K. Moon <kmoon@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798785}