Add UMA tracing to requestIdleCallback.

Source/core/dom/ScriptedIdleTaskController.cpp

Index: Source/core/dom/ScriptedIdleTaskController.cpp
diff --git a/Source/core/dom/ScriptedIdleTaskController.cpp b/Source/core/dom/ScriptedIdleTaskController.cpp
index 34b65b67840c7d0f1b3c7f15adeeba7604da28d3..d145f6dc2ed46bea77f154d882ddbb13b4351eb1 100644
--- a/Source/core/dom/ScriptedIdleTaskController.cpp
+++ b/Source/core/dom/ScriptedIdleTaskController.cpp
@@ -119,8 +119,14 @@ void ScriptedIdleTaskController::runCallback(CallbackId id, double deadlineSecon
     if (!callback)
         return;
 
+    double allotedTimeMillis = std::max((deadlineSeconds - monotonicallyIncreasingTime()) * 1000, 0.0);
+    Platform::current()->histogramCustomCounts("WebCore.ScriptedIdleTaskController.IdleCallbackDeadline", allotedTimeMillis, 0, 50, 50);

[jochen (gone - plz use gerrit), 2015/09/15 16:38:46]

that looks like it's in seconds?

[rmcilroy, 2015/09/15 16:50:46]

I don't think so. The seconds values are multiplied by 1000 above to make ms
(the numbers also looked right in local testing).

[jochen (gone - plz use gerrit), 2015/09/15 16:53:37]

hum, ok, so you expect that there will be at most 50ms alloted ever?

[Sami, 2015/09/15 17:01:04]

Right, that 50ms maximum required by the spec too.

[rmcilroy, 2015/09/15 17:03:23]

Yup, the spec explicitly limits the maximum deadline to 50ms (to try and ensure
user input is always responded to within ~100ms and appear instant). This is
also the case for internal idle tasks (e.g. those used by V8's GC).

The overrun could be any amount of time, so I've got a 10 second max there, but
here 50ms should be the most we ever see.

+
     // TODO(rmcilroy): Add devtools tracing.
     callback->handleEvent(IdleDeadline::create(deadlineSeconds, callbackType));
+
+    double overrunMillis = std::max((monotonicallyIncreasingTime() - deadlineSeconds) * 1000, 0.0);
+    Platform::current()->histogramCustomCounts("WebCore.ScriptedIdleTaskController.idleCallbackOverrunMs", overrunMillis, 0, 10000, 50);

[Sami, 2015/09/15 16:58:44]

"idle" isn't capitalized here but is above -- I guess these are generally always
capitalized? This one also has "Ms" in the end but the other doesn't.

[rmcilroy, 2015/09/15 17:05:22]

Argh, changed this locally but didn't upload. Thanks, I'll fix when back at a
computer.

[rmcilroy, 2015/09/15 17:19:50]

Done.

 }
 
 void ScriptedIdleTaskController::stop()