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This CL migrates `deferred_invalidates_` to PdfViewPluginBase so that
Invalidate() and InvalidateAfterPaintDone() can be migrated as well,
which will allow these two methods to be shared by both
OutOfProcessInstance and PdfViewWebPlugin.

This CL also removed PdfViewPluginBase::in_paint() since all uses of
`in_paint_` are within the same class.

Bug: 1140629
Change-Id: I819687e402b3a0687b597194e2210c9282772c22
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2639169
Commit-Queue: Hui Yingst <nigi@chromium.org>
Reviewed-by: K. Moon <kmoon@chromium.org>
Reviewed-by: Daniel Hosseinian <dhoss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#846398}