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 11ae958f7a0f39ca8f319a4afb6316f184005c50..3e0fa1692f18b2160f03a801f618b8abbabc43fe 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -265,13 +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.

[Ilya Sherman, 2016/12/17 00:57:12]

Hmm, this seems like it might be a bit confusing to work with, if there are any
flags that users regularly toggle on and off again.  Is that a concern?

[dmazzoni, 2016/12/19 22:51:51]

I clarified in the comment that the flags are flipped on automatically,
and they can only be flipped off using an internal page, so it's not my goal to
track that right now. Perhaps I could also add an action.

+  </summary>
+</histogram>
+
 <histogram name="Accessibility.State" enum="BooleanEnabled">
   <owner>dmazzoni@chromium.org</owner>
   <owner>kenjibaheux@google.com</owner>
   <summary>
     Whether Chrome has enabled accessibility support because it detects
     supported assistive technology running, or due to being manually enabled via
-    a command-line flag (checked once 45 secs after startup).
+    a command-line flag (checked once 45 secs after startup). NOTE:
+    Accessibility.ModeFlag is potentially more useful.

[Ilya Sherman, 2016/12/17 00:57:12]

Is the new metrics more useful enough to where we should deprecate this one?

[dmazzoni, 2016/12/19 22:51:51]

Sure, done. The main reason to keep it a bit longer is for a bit of continuity.
After a few releases we can stop tracking Accessibility.State.

   </summary>
 </histogram>
 
@@ -75228,6 +75238,17 @@ 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 mdoe flags that are enabled.</summary>

[Ilya Sherman, 2016/12/17 00:57:12]

nit: s/mdoe/mode

[dmazzoni, 2016/12/19 22:51:51]

Done.

+  <int value="0" label="UMA_AX_MODE_FLAG_NATIVE_APIS">Native APIs</int>

[Ilya Sherman, 2016/12/17 00:57:12]

nit: The "Native APIs" text is only shown on hover, and is pretty clear from the
label anyway.  I'd write this as either

  <int value="0" label="Native APIs"/>

or

  <int value="0" label="UMA_AX_MODE_FLAG_NATIVE_APIS"/>

[dmazzoni, 2016/12/19 22:51:51]

Sounds good, thanks.

+  <int value="1" label="UMA_AX_MODE_FLAG_WEB_CONTENTS">Web Contents</int>
+  <int value="2" label="UMA_AX_MODE_FLAG_INLINE_TEXT_BOXES">
+    Inline Text Boxes
+  </int>
+  <int value="3" label="UMA_AX_MODE_FLAG_SCREEN_READER">Screen Reader</int>
+  <int value="4" label="UMA_AX_MODE_FLAG_HTML">HTML</int>
+</enum>
+
 <enum name="AccessibilityWinAPIEnum" type="int">
   <summary>
     Track which Windows accessibility APIs are being called by clients.