Cache Backend Proxy to intercept all cache events from the IO thread.

net/net.gyp

Index: net/net.gyp
diff --git a/net/net.gyp b/net/net.gyp
index 5a3e615e957326c565af0f97ed9fb24dd2491df6..a83f957828c3baed1f76cc958cc6d8f70dae6ddb 100644
--- a/net/net.gyp
+++ b/net/net.gyp
@@ -7,6 +7,7 @@
     'chromium_code': 1,
 
     'linux_link_kerberos%': 0,
+    'use_tracing_cache_backend%': 0,
     'conditions': [
       ['chromeos==1 or OS=="android" or OS=="ios"', {
         # Disable Kerberos on ChromeOS, Android and iOS, at least for now.
@@ -373,6 +374,8 @@
         'disk_cache/stress_support.h',
         'disk_cache/trace.cc',
         'disk_cache/trace.h',
+        'disk_cache/tracing_cache_backend.cc',
+        'disk_cache/tracing_cache_backend.h',
         'disk_cache/simple/simple_backend_impl.cc',
         'disk_cache/simple/simple_backend_impl.h',
         'disk_cache/simple/simple_disk_format.cc',
@@ -1111,6 +1114,11 @@
             'dns/dns_client.cc',
           ],
         }],
+        ['use_tracing_cache_backend==1', {
+          'defines': [
+            'USE_TRACING_CACHE_BACKEND'
+          ],
+         }],
         ['use_openssl==1', {
             'sources!': [
               'base/crypto_module_nss.cc',