Untangle PdfGetSaveDataInBlocks and PdfUseShowSaveFilePicker flags.

Make 'PdfGetSaveDataInBlocks' flag independent of
'PdfUseShowSaveFilePicker' by adding support of the old save file picker
when `PdfUseShowSaveFilePicker` is disabled.
This will help with the separate launch of the two features.

To do so, update `FileSystem` definitions to expose 3 required events.
The update can be reverted when `PdfUseShowSaveFilePicker` is launched.

Bug: 394111292
Change-Id: I9c279f03842b86292568e69dcbde5f795b8f5621
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6939710
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Andy Phan <andyphan@chromium.org>
Commit-Queue: Ramin Halavati <rhalavati@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1515860}