Allow folks to enable File System API with a Flag

[pes10k]

could i bug you to change the Native File System API feature from “off with no way to enable” (what it is currently) to “disabled by default but enable-able with a flag”?

[pikilon]

Yeah or even as a site config feature

[mkarolin]

Test plan:

1. Start Brave
2. Navigate to https://brave.com
3. Right-click and select Inspect to open DevTools
4. In the DevTools, select Console tab
5. Paste the following code in the Console window: self.showOpenFilePicker()

Expected result:
Uncaught TypeError: self.showOpenFilePicker is not a function

6. Navigate to brave://flags
7. In the search box type file-system-access-api

Expected result:
File System Access API item is shown

8. Change the File System API item's status to Enabled
9. Click on the Relaunch button
10. Once the browser restarts, repeat steps 2 - 5

Expected result:
Open file picker dialog is shown

[mkarolin]

Note: Removed OS/Andrdoid label as File System Access API is not available for Android yet. See: https://bugs.chromium.org/p/chromium/issues/detail?id=1011535

[stephendonner]

Verified PASSED using

Brave	1.33.51 Chromium: 95.0.4638.69 (Official Build) nightly (x86_64)
Revision	6a1600ed572fedecd573b6c2b90a22fe6392a410-refs/branch-heads/4638@{#984}
OS	macOS Version 11.6.1 (Build 20G224)

Steps:

1. Start Brave
2. Navigate to https://brave.com
3. Right-click and select Inspect to open DevTools
4. In the DevTools, select Console tab
5. Paste the following code in the Console window: self.showOpenFilePicker()

Expected result:
Uncaught TypeError: self.showOpenFilePicker is not a function

6. Navigate to brave://flags
7. In the search box type file-system-access-api

Expected result:
File System Access API item is shown

example	example

8. Change the File System API item's status to Enabled
9. Click on the Relaunch button
10. Once the browser restarts, repeat steps 2 - 5

Expected result:
Open file picker dialog is shown