pdf: Use chrome.fileSystem.chooseFile when saving PDF from extension

This matches the save behavior of the plugin, where a file picker
is opened rather than the file being immediately downloaded.

Bug: 61248
Change-Id: I2481ffcb8fe1ba346acadb8106d786ee09fc8a94
Reviewed-on: https://chromium-review.googlesource.com/c/1356161
Commit-Queue: dstockwell <dstockwell@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Ben Wells <benwells@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623197}
3 files changed