Add perf histogram and tests for inline signin

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 a8ccf0229b1a109941ae667b6767d2cc6d158e5d..566bf303efd6140f6f5b4e03c640de9941f16311 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -17336,6 +17336,20 @@ other types of suffix sets.
   </summary>
 </histogram>
 
+<histogram name="Signin.InlineUI.LoadAfterStartupDuration" units="milliseconds">
+  <summary>
+    How long it takes to load the inline login UI after startup, including the
+    gaia webview or iframe.
+  </summary>
+</histogram>
+
+<histogram name="Signin.InlineUI.LoadOnStartupDuration" units="milliseconds">
+  <summary>
+    How long it takes to load the inline login UI on startup, including the gaia
+    webview or iframe.

[jar (doing other things), 2014/01/24 22:52:14]

Can you expand on the prose to better distinguish the pair of histograms?  I
could barely find the difference in the text.

If you have to write a ton to be clearer, you *could* use a field trial to have
shared prose here, and the distinctive prose in the field trial for the two
cases... but it is not that big a win with the current minimalist prose.

+  </summary>
+</histogram>
+
 <histogram name="Signin.OneClickConfirmation" enum="SigninFlowConfirmations">
   <summary>
     Count of the ways users interact with the confirmation dialogs of the new