[fuchsia] Support "Save as PDF"

Fuchsia does not have a system dialog yet to choose the save location.
If uses tries to save pdf from print preview, chrome will crash.

This CL implemented the workaround for Fuchsia proposed in
crbug.com/1226242, where we store a pdf file directly to the default
path determined by the download preferences.

BUG=1393183
TEST=unit_tests

Change-Id: I587ec1e1492290f302875fe9813832d210fa8afa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4190690


Reviewed-by: Bryant Chandler <bryantchandler@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Xinan Lin <linxinan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1112868}