Make the blockfile cache an optional build product for chrome target.

net/net.gyp

Index: net/net.gyp
diff --git a/net/net.gyp b/net/net.gyp
index f4c1dec8e61cb8b18d1a43daeb2c35969c374527..680569a9a8282ee12567b54d11216e93c2832e4b 100644
--- a/net/net.gyp
+++ b/net/net.gyp
@@ -7,6 +7,7 @@
     'chromium_code': 1,
 
     'linux_link_kerberos%': 0,
+    'use_blockfile_cache_backend%': 1,
     'use_tracing_cache_backend%': 0,
     'conditions': [
       ['chromeos==1 or embedded==1 or OS=="android" or OS=="ios"', {
@@ -1336,11 +1337,18 @@
             'dns/dns_client.cc',
           ],
         }],
+	['use_blockfile_cache_backend==1', {
+	  'defines': [
+	    'USE_BLOCKFILE_CACHE_BACKEND',
+	  ],
+	}, { # use_blockfile_cache_backend == 0
+	  'sources/': [ ['exclude', '^disk_cache/blockfile/' ] ]
+        }],
         ['use_tracing_cache_backend==1', {
           'defines': [
             'USE_TRACING_CACHE_BACKEND'
           ],
-         }],
+        }],
         ['use_openssl==1', {
             'sources!': [
               'base/crypto_module_nss.cc',