Split SVGImage-related cases into separate UseCounter histograms

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 41f3ecd9d823dc6becd0a4e73b611f14f5d0bc9e..bb257bfd9467a28f9ffa2d6efaee2d4571542005 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -66459,6 +66459,35 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </details>
 </histogram>
 
+<histogram name="WebCore.UseCounter_TEST.SVGImage.CSSProperties"
+    enum="MappedCSSProperties">
+  <owner>rbyers@chromium.org</owner>
+  <summary>
+    Like WebCore.UseCounter_TEST.CSSProperties but specifically for the case of
+    CSS properties used inside of an SVG image.
+  </summary>
+  <details>
+    The bucket numbered '1' is incremented each time a new SVG image is created.

[rkaplow, 2016/09/12 15:20:57]

this description is a bit confused since this uses a enum. WHy is this described
like this?

[Rick Byers, 2016/09/12 21:04:05]

Yeah I was just being lazy copying the existing text rather than improving it. 
Fixed, and fixed the source I copied from (which was itself copied from the
legacy description but I'll remove that soon so didn't bother improving that).

+    Note that the same SVG image can be used across multiple tabs in a single
+    renderer but this counts as a single usage.  See http://crbug.com/236262.
+  </details>
+</histogram>
+
+<histogram name="WebCore.UseCounter_TEST.SVGImage.Features"
+    enum="FeatureObserver">
+  <owner>rbyers@chromium.org</owner>
+  <summary>
+    Like WebCore.UseCounter_TEST.Features except specifically for the case of
+    SVG Images.
+  </summary>
+  <details>
+    Count of how many SVG images use various features. The PageVisits bucket is
+    incremented each time a new SVG image is created.  Note that the same SVG
+    image can be used across multiple tabs in a single renderer but this counts
+    as a single usage.  See http://crbug.com/236262.
+  </details>
+</histogram>
+
 <histogram name="WebCore.V8DOMWindowShell.createContext.IsolatedWorld"
     units="ms">
   <obsolete>