Enable persistent filesystem in incognito mode.

Persistent filesystem is enabled by default in incognito mode. Storage
is still done in memory and is deleted when browsing session ends.

Tested:
third_party/blink/tools/run_blink_wptserve.py
out/Default/chrome --incognito \
http://localhost:8001/wpt_internal/file-system-api\
/temporary_vs_persistent.window.html

Bug: 990592
Change-Id: I2294dc65c79553917c28541897d5018afaa7b678
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1901079
Commit-Queue: Ramin Halavati <rhalavati@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713351}

storage/browser/file_system/file_system_features.cc

@@ -10,7 +10,7 @@ namespace features {
 
 // Enables persistent Filesystem API in incognito mode.
 const base::Feature kEnablePersistentFilesystemInIncognito{
-    "EnablePersistentFilesystemInIncognito", base::FEATURE_DISABLED_BY_DEFAULT};
+    "EnablePersistentFilesystemInIncognito", base::FEATURE_ENABLED_BY_DEFAULT};
 }  // namespace features
 
 }  // namespace storage
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