QUIC - Record loading of QUICServerInfo from disk cache.

Added Net.QuicServerInfo.DiskCacheLoadTime histogram to track the read
time.

R=asvitkine@chromium.org, rch@chromium.org

Review URL: https://codereview.chromium.org/674453003

Cr-Commit-Position: refs/heads/master@{#300710}

net/http/disk_cache_based_quic_server_info.cc

@@ -78,6 +78,7 @@ DiskCacheBasedQuicServerInfo::DiskCacheBasedQuicServerInfo(
 void DiskCacheBasedQuicServerInfo::Start() {
   DCHECK(CalledOnValidThread());
   DCHECK_EQ(GET_BACKEND, state_);
+  load_start_time_ = base::TimeTicks::Now();
   DoLoop(OK);
 }
 
@@ -312,6 +313,8 @@ int DiskCacheBasedQuicServerInfo::DoWaitForDataReadyDone() {
   }
   entry_ = NULL;
   Parse(data_);
+  UMA_HISTOGRAM_TIMES("Net.QuicServerInfo.DiskCacheLoadTime",
+                      base::TimeTicks::Now() - load_start_time_);
   return OK;
 }
 

net/http/disk_cache_based_quic_server_info.h

@@ -10,6 +10,7 @@
 #include "base/memory/ref_counted.h"
 #include "base/memory/weak_ptr.h"
 #include "base/threading/non_thread_safe.h"
+#include "base/time/time.h"
 #include "net/base/completion_callback.h"
 #include "net/disk_cache/disk_cache.h"
 #include "net/quic/crypto/quic_server_info.h"
@@ -98,6 +99,7 @@ class NET_EXPORT_PRIVATE DiskCacheBasedQuicServerInfo
   scoped_refptr<IOBuffer> read_buffer_;
   scoped_refptr<IOBuffer> write_buffer_;
   std::string data_;
+  base::TimeTicks load_start_time_;
 
   base::WeakPtrFactory<DiskCacheBasedQuicServerInfo> weak_factory_;
 };

tools/metrics/histograms/histograms.xml

@@ -17214,6 +17214,11 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="Net.QuicServerInfo.DiskCacheLoadTime" units="milliseconds">
+  <owner>rtenneti@chromium.org</owner>
+  <summary>Time spent to load QUIC server information from disk cache.</summary>
+</histogram>
+
 <histogram name="Net.QuicServerInfo.DiskCacheReadTime" units="milliseconds">
   <obsolete>
     Deprecated as of 10/2014. Replaced by DiskCacheWaitForDataReadyTime.