Adds UMA histograms to better understand image alt usage for a11y.

This logs image minimum dimension and dimension ratio for all images
loaded when a screen reader is enabled. For more info see go/gchro.

Bug: 905419
Change-Id: I75402356c4cf5e64211e37d928634b7b4754fc2b
Reviewed-on: https://chromium-review.googlesource.com/c/1336001
Commit-Queue: Katie Dektar <katie@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608922}

content/renderer/accessibility/render_accessibility_impl.cc

@@ -11,6 +11,7 @@
 #include "base/containers/queue.h"
 #include "base/location.h"
 #include "base/memory/ptr_util.h"
+#include "base/metrics/histogram_macros.h"
 #include "base/single_thread_task_runner.h"
 #include "base/strings/utf_string_conversions.h"
 #include "base/threading/thread_task_runner_handle.h"
@@ -436,6 +437,9 @@ void RenderAccessibilityImpl::SendPendingAccessibilityEvents() {
   // If there's a layout complete message, we need to send location changes.
   bool had_layout_complete_messages = false;
 
+  // If there's a load complete message, we need to send image metrics.
+  bool had_load_complete_messages = false;
+
   ScopedFreezeBlinkAXTreeSource freeze(&tree_source_);
 
   // Loop over each event and generate an updated event message.
@@ -444,6 +448,9 @@ void RenderAccessibilityImpl::SendPendingAccessibilityEvents() {
     if (event.event_type == ax::mojom::Event::kLayoutComplete)
       had_layout_complete_messages = true;
 
+    if (event.event_type == ax::mojom::Event::kLoadComplete)
+      had_load_complete_messages = true;
+
     auto obj = WebAXObject::FromWebDocumentByID(document, event.id);
 
     // Make sure the object still exists.
@@ -519,6 +526,9 @@ void RenderAccessibilityImpl::SendPendingAccessibilityEvents() {
 
     bundle.updates.push_back(update);
 
+    if (had_load_complete_messages)
+      RecordImageMetrics(&update);
+
     VLOG(1) << "Accessibility tree update:\n" << update.ToString();
   }
 
@@ -899,4 +909,49 @@ void RenderAccessibilityImpl::ScrollPlugin(int id_to_make_visible) {
       WebRect(bounds.x(), bounds.y(), bounds.width(), bounds.height()));
 }
 
+void RenderAccessibilityImpl::RecordImageMetrics(AXContentTreeUpdate* update) {
+  if (!render_frame_->accessibility_mode().has_mode(ui::AXMode::kScreenReader))
+    return;
+  float scale_factor = render_frame_->GetRenderView()->GetDeviceScaleFactor();
+  for (size_t i = 0; i < update->nodes.size(); ++i) {
+    ui::AXNodeData& node_data = update->nodes[i];
+    if (node_data.role != ax::mojom::Role::kImage)
+      continue;
+    // Convert to DIPs based on screen scale factor.
+    int width = node_data.relative_bounds.bounds.width() / scale_factor;
+    int height = node_data.relative_bounds.bounds.height() / scale_factor;
+    if (width == 0 || height == 0)
+      continue;
+    // We log the min size in a histogram with a max of 10000, so set a ceiling
+    // of 10000 on min_size.
+    int min_size = std::min(std::min(width, height), 10000);
+    int max_size = std::max(width, height);
+    // The ratio is always the smaller divided by the larger so as not to go
+    // over 100%.
+    int ratio = min_size * 100.0 / max_size;
+    const std::string name =
+        node_data.GetStringAttribute(ax::mojom::StringAttribute::kName);
+    bool explicitly_empty = node_data.GetNameFrom() ==
+                            ax::mojom::NameFrom::kAttributeExplicitlyEmpty;
+    if (!name.empty()) {
+      UMA_HISTOGRAM_PERCENTAGE(
+          "Accessibility.ScreenReader.Image.SizeRatio.Labeled", ratio);
+      UMA_HISTOGRAM_COUNTS_10000(
+          "Accessibility.ScreenReader.Image.MinSize.Labeled", min_size);
+    } else if (explicitly_empty) {
+      UMA_HISTOGRAM_PERCENTAGE(
+          "Accessibility.ScreenReader.Image.SizeRatio.ExplicitlyUnlabeled",
+          ratio);
+      UMA_HISTOGRAM_COUNTS_10000(
+          "Accessibility.ScreenReader.Image.MinSize.ExplicitlyUnlabeled",
+          min_size);
+    } else {
+      UMA_HISTOGRAM_PERCENTAGE(
+          "Accessibility.ScreenReader.Image.SizeRatio.Unlabeled", ratio);
+      UMA_HISTOGRAM_COUNTS_10000(
+          "Accessibility.ScreenReader.Image.MinSize.Unlabeled", min_size);
+    }
+  }
+}
+
 }  // namespace content

content/renderer/accessibility/render_accessibility_impl.h

@@ -137,6 +137,7 @@ class CONTENT_EXPORT RenderAccessibilityImpl
   void ScrollPlugin(int id_to_make_visible);
   ax::mojom::EventFrom GetEventFrom();
   void ScheduleSendAccessibilityEventsIfNeeded();
+  void RecordImageMetrics(AXContentTreeUpdate* update);
 
   // The RenderFrameImpl that owns us.
   RenderFrameImpl* render_frame_;

tools/metrics/histograms/histograms.xml

@@ -403,6 +403,34 @@ uploading your change for review.
   </summary>
 </histogram>
 
+<histogram name="Accessibility.ScreenReader.Image.MinSize" units="px"
+    expires_after="2019-04-01">
+<!-- Name completed by histogram_suffixes
+       name="AccessibilityScreenReaderImage" -->
+
+  <owner>katie@chromium.org</owner>
+  <owner>dmazzoni@chromium.org</owner>
+  <summary>
+    For every image on a page with screen reader accessibility mode enabled,
+    this histogram will be used to understand minimum dimension of the image,
+    i.e. either the width or height in px.
+  </summary>
+</histogram>
+
+<histogram name="Accessibility.ScreenReader.Image.SizeRatio" units="%"
+    expires_after="2019-04-01">
+<!-- Name completed by histogram_suffixes
+       name="AccessibilityScreenReaderImage" -->
+
+  <owner>katie@chromium.org</owner>
+  <owner>dmazzoni@chromium.org</owner>
+  <summary>
+    For every image on a page with screen reader accessibility mode enabled,
+    this histogram will be used to understand the size ratio of the image. This
+    is the smaller dimension divided by the larger dimension.
+  </summary>
+</histogram>
+
 <histogram name="Accessibility.State" enum="BooleanEnabled">
   <obsolete>
     Deprecated 12/2016 in Issue 672205 with the addition of
@@ -125641,6 +125669,15 @@ uploading your change for review.
 
 <histogram_suffixes_list>
 
+<histogram_suffixes name="AccessibilityScreenReaderImage" separator=".">
+  <suffix base="true" name="ExplicitlyUnlabeled"
+      label="explicitly unlabeled image"/>
+  <suffix base="true" name="Labeled" label="labeled image"/>
+  <suffix name="Unlabeled" label="unlabeled image"/>
+  <affected-histogram name="Accessibility.ScreenReader.Image.MinSize"/>
+  <affected-histogram name="Accessibility.ScreenReader.Image.SizeRatio"/>
+</histogram_suffixes>
+
 <histogram_suffixes name="AccessorySheetType" separator=".">
   <suffix name="Passwords" label="Password suggestions and generation."/>
   <affected-histogram name="KeyboardAccessory.AccessorySheetSuggestionCount"/>