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This change adds a writable stream output to the `FileSystemFileHandle`.

This follows some
[discussion](https://github.com/WICG/native-file-system/issues/19#issuecomment-490579093) on github.

While this writable stream does not yet feature stream-like behavior, it
carries over some of the interface from `NativeFileSystemWriter`, namely
`truncate` and `write`.

Bug: 955189
Change-Id: I8b8c9ff1ef36adfb30501251563699c6c7973889
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1623534Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: Jeremy Roman <jbroman@chromium.org>
Auto-Submit: Olivier Yiptong <oyiptong@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662620}