Add UMA for how often the first read sniffs the final mime type.

This is to figure out whether we need to buffer data in the network service's URLLoaderImpl.

BUG=746144

Change-Id: I10a0b5923b8d9fefdceee98bbc07671e53ab4f8d
Reviewed-on: https://chromium-review.googlesource.com/581742
Commit-Queue: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488776}

content/browser/loader/mime_sniffing_resource_handler.cc

@@ -126,6 +126,7 @@ MimeSniffingResourceHandler::MimeSniffingResourceHandler(
       must_download_is_set_(false),
       read_buffer_size_(0),
       bytes_read_(0),
+      sniffed_(false),
       parent_read_buffer_(nullptr),
       parent_read_buffer_size_(nullptr),
       intercepting_handler_(intercepting_handler),
@@ -135,7 +136,13 @@ MimeSniffingResourceHandler::MimeSniffingResourceHandler(
       weak_ptr_factory_(this) {
 }
 
-MimeSniffingResourceHandler::~MimeSniffingResourceHandler() {}
+MimeSniffingResourceHandler::~MimeSniffingResourceHandler() {
+  if (sniffed_) {
+    UMA_HISTOGRAM_BOOLEAN(
+        "Net.MimeSniffDeterminedFromFirstRead",
+        first_sniffed_mime_type_ == response_->head.mime_type);
+  }
+}
 
 void MimeSniffingResourceHandler::OnWillStart(
     const GURL& url,
@@ -276,6 +283,10 @@ void MimeSniffingResourceHandler::OnReadCompleted(
   bool made_final_decision =
       net::SniffMimeType(read_buffer_->data(), bytes_read_, request()->url(),
                          type_hint, &new_type);
+  if (!sniffed_) {
+    first_sniffed_mime_type_ = new_type;
+    sniffed_ = true;
+  }
 
   // SniffMimeType() returns false if there is not enough data to determine
   // the mime type. However, even if it returns false, it returns a new type

content/browser/loader/mime_sniffing_resource_handler.h

@@ -175,6 +175,10 @@ class CONTENT_EXPORT MimeSniffingResourceHandler
   int read_buffer_size_;
   int bytes_read_;
 
+  // The mime type that was sniffed the first time we had data.
+  bool sniffed_;
+  std::string first_sniffed_mime_type_;
+
   // Pointers to parent-owned read buffer and its size.  Only used for first
   // OnWillRead call.
   scoped_refptr<net::IOBuffer>* parent_read_buffer_;

tools/metrics/histograms/histograms.xml

@@ -38850,6 +38850,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Net.MimeSniffDeterminedFromFirstRead" enum="Boolean">
+  <owner>jam@chromium.org</owner>
+  <summary>
+    True counts the events when mime sniffing succeeded from the first read.
+    False means that it took more reads to determine the mime type.
+  </summary>
+</histogram>
+
 <histogram name="Net.MTPR_GetProxyForUrl_Thread_Wait_Time" units="ms">
   <obsolete>
     Removed in Chrome 39.