Opt-in UKM for XSrcsetDescriptor and WSrcsetDescriptor.

Enabled UKM collection for the XSrcset and WSrcset tags for image
srcset. See Opt-in instructions here:
https://cs.chromium.org/chromium/src/docs/use_counter_wiki.md

Srcset usage is fairly low but is likely above the 1% recommended
threshold. However, I think several of the other features counted
are also higher. Would like to have this available as an experiment
for using lower resolution image versions for data savers is run.

Bug: 1059444
Change-Id: Id2fba387d85f2c6d8537002d8cf5f6b0e06e36dc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2092184Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Commit-Queue: Michael Crouse <mcrouse@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748246}

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

@@ -125,6 +125,8 @@ UseCounterPageLoadMetricsObserver::GetAllowedUkmFeatures() {
           WebFeature::kRequestedFileSystemTemporary,
           WebFeature::kRequestedFileSystemPersistent,
           WebFeature::kSMSReceiverStart,
+          WebFeature::kSrcsetWDescriptor,
+          WebFeature::kSrcsetXDescriptor,
           WebFeature::kRequestFileSystem,
           WebFeature::kBaseElement,
           WebFeature::kBaseWithCrossOriginHref,