[FS] Add UKM use counters for File System Access API.

This adds use counters for the entry points to the File System Access
API (https://wicg.github.io/file-system-access/) to UKM use counters to
record usage of the API and observe how it is being used to help with
improvements to the shape of the API.

Change-Id: I31a0491771f31a14b86c6f29fff42457e1503072
Bug: 1134624
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2438678
Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813271}

components/page_load_metrics/browser/observers/use_counter/ukm_features.cc

@@ -178,6 +178,11 @@ UseCounterPageLoadMetricsObserver::GetAllowedUkmFeatures() {
           WebFeature::kAddressSpacePrivateEmbeddedInPublicNonSecureContext,
           WebFeature::kAddressSpacePrivateEmbeddedInUnknownSecureContext,
           WebFeature::kAddressSpacePrivateEmbeddedInUnknownNonSecureContext,
+          WebFeature::kFileSystemPickerMethod,
+          WebFeature::kV8Window_ShowOpenFilePicker_Method,
+          WebFeature::kV8Window_ShowSaveFilePicker_Method,
+          WebFeature::kV8Window_ShowDirectoryPicker_Method,
+          WebFeature::kV8StorageManager_GetDirectory_Method,
       }));
   return *opt_in_features;
 }