Refactor PdfViewWebPlugin save data code to support multiple buffers

The save to Drive feature will use a similar method as
`SaveBlockToBuffer` to get saved data in blocks. However, Save to Drive
has some additional requirements:
- Be able to save to Drive while saving to disk is in progress
- Be able to save a newly revised PDF to disk while the previous
  revision is still being uploaded to Drive.

To meet these requirements, the `SaveBlockToBuffer()` method is
refactored into `SaveBlockToBufferImpl()`. It accepts a buffer reference
as a parameter to support multiple buffers.

Bug: 433496885
Change-Id: I318f5baa4eb4a2226ec42b4caa50bacd47dd9c23
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6779170
Reviewed-by: Andy Phan <andyphan@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1495780}