Add metrics for coalesced permission prompts.

chrome/browser/permissions/permission_uma_util.cc

Index: chrome/browser/permissions/permission_uma_util.cc
diff --git a/chrome/browser/permissions/permission_uma_util.cc b/chrome/browser/permissions/permission_uma_util.cc
index 8d38d0451210d25d95cecac7af49fd236e0d2b1b..07d34c34db251448968f81055f5980459b7ec82d 100644
--- a/chrome/browser/permissions/permission_uma_util.cc
+++ b/chrome/browser/permissions/permission_uma_util.cc
@@ -286,4 +286,19 @@ void PermissionUmaUtil::PermissionPromptShown(
       "Permissions.Prompt.Shown",
       static_cast<base::HistogramBase::Sample>(permission_prompt_type),
       static_cast<base::HistogramBase::Sample>(PermissionBubbleType::NUM));
+
+  UMA_HISTOGRAM_ENUMERATION(
+      "Permissions.Prompt.RequestsPerPrompt",
+      static_cast<base::HistogramBase::Sample>(requests.size()),
+      static_cast<base::HistogramBase::Sample>(10));
+
+  if (requests.size() > 1) {
+    for (const auto* request : requests) {
+      UMA_HISTOGRAM_ENUMERATION(
+          "Permissions.Prompt.MergedBubbleTypes",
+          static_cast<base::HistogramBase::Sample>(
+              request->GetPermissionBubbleType()),

[felt, 2016/04/28 23:55:19]

i am wondering, how do you plan to interpret this? if we end up with a histogram
full of camera, mic, and geolocation, we still won't know which of them are
being requested together right?

[benwells, 2016/04/29 00:07:51]

That's right. We'd know that camera, mic and geo often get merged into other
things, but not what they get merged into.

We could explode out the combinations we report into one histogram enum:
mic+camera
mic+geo
camera+geo
mic+camera+geo

But I thought that seemed pretty complicated and I'm not sure its worth it.

[felt, 2016/04/29 00:19:43]

if you are looking for specific combinations (like mic+camera) you could
specifically histogram those instead of everything. (actually my honest guess is
this will all be mic+camera anyway.)

+          static_cast<base::HistogramBase::Sample>(PermissionBubbleType::NUM));
+    }
+  }
 }