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Add message handler HandleSaveAttachmentMessage() for downloading PDF
attachments.

As the caller of PDFiumEngine::GetAttachmentData(),
HandleSaveAttachmentMessage() validates the attachment's index,
readability and whether the size is non-zero before calling, and checks
whether the returned attachment's size matches the stored attachment
size in |doc_attachment_info_list_|.

After calling GetAttachmentData() to fetch the attachment's data,
HandleSaveAttachmentMessage() posts a reply to save the data to
disk.

Bug: 177188
Change-Id: I34ad386447cdcb4025d0f545e4ff8b9d3b818659
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2295998
Commit-Queue: Hui Yingst <nigi@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798865}