Add support for faster, more limited accessibility modes.

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index 32349fca2bfbaf994f06b8a32680bdc77ecda2c7..6bf9f8d26f391d46f3406205916fd7a1986ab31e 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -265,7 +265,23 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Accessibility.ModeFlag" enum="AccessibilityModeFlagEnum">
+  <owner>dmazzoni@chromium.org</owner>
+  <owner>aboxhall@chromium.org</owner>
+  <summary>
+    Whether individual accessibility mode flags are set. Tracked when each mode
+    flag is flipped from false to true. These flags are flipped automatically
+    when software communicates with Chrome via accessibility APIs. Flags can
+    only be flipped off by advanced users or for debugging using
+    chrome://accessibility/ - and that isn't tracked in this histogram.
+  </summary>
+</histogram>
+
 <histogram name="Accessibility.State" enum="BooleanEnabled">
+  <obsolete>
+    Deprecated 12/2016 in Issue 672205 with the addition of
+    Accessibility.ModeFlag.
+  </obsolete>
   <owner>dmazzoni@chromium.org</owner>
   <owner>kenjibaheux@google.com</owner>
   <summary>
@@ -75298,6 +75314,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="2" label="ScrolledMainFrameWithUnacceleratedFixedRootBackground"/>
 </enum>
 
+<enum name="AccessibilityModeFlagEnum" type="int">
+  <summary>Track individual accessibility mode flags that are enabled.</summary>
+  <int value="0" label="Native APIs"/>
+  <int value="1" label="Web Contents"/>
+  <int value="2" label="Inline Text Boxes"/>
+  <int value="3" label="Screen Reader"/>
+  <int value="4" label="HTML"/>
+</enum>
+
 <enum name="AccessibilityWinAPIEnum" type="int">
   <summary>
     Track which Windows accessibility APIs are being called by clients.