Add histogram to capture percentage of users using SODA

This CL adds a histogram metric to capture the percentage of Live caption users using the Speech On-Device API(SODA).

Bug: 1159105
Change-Id: Ia67c60b59c5f5f9e670c8a8e531c11cbf40f7e52
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2594171Reviewed-by: Chris Thompson <cthomp@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Evan Liu <evliu@google.com>
Cr-Commit-Position: refs/heads/master@{#839031}

chrome/browser/accessibility/caption_controller.cc

@@ -64,6 +64,9 @@ void CaptionController::Init() {
   if (!base::FeatureList::IsEnabled(media::kLiveCaption))
     return;
 
+  base::UmaHistogramBoolean(
+      "Accessibility.LiveCaption.UseSodaForLiveCaption",
+      base::FeatureList::IsEnabled(media::kUseSodaForLiveCaption));
   pref_change_registrar_ = std::make_unique<PrefChangeRegistrar>();
   pref_change_registrar_->Init(profile_->GetPrefs());
   auto* command_line = base::CommandLine::ForCurrentProcess();

tools/metrics/histograms/histograms_xml/accessibility/histograms.xml

@@ -677,6 +677,18 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
   </summary>
 </histogram>
 
+<histogram name="Accessibility.LiveCaption.UseSodaForLiveCaption"
+    enum="BooleanEnabled" expires_after="2021-04-30">
+  <owner>abigailbklein@google.com</owner>
+  <owner>evliu@google.com</owner>
+  <owner>chrome-a11y-core@google.com</owner>
+  <summary>
+    Whether the Live Caption feature is powered by the Speech On-Device API
+    (SODA). This is logged once during the initialization of the caption
+    controller at browser start up.
+  </summary>
+</histogram>
+
 <histogram name="Accessibility.LiveCaption.WebsiteBlocked"
     enum="BooleanEnabled" expires_after="2021-04-30">
   <owner>katie@chromium.org</owner>