Fix link error when safe browsing is disabled.

This was breaking the 'chrome-linux-redux' bot, which compiles Chrome
for Linux with the same flags we're using for the Android port.

BUG=None
TEST=Compiles


Review URL: http://codereview.chromium.org/9867002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@129463 0039d316-1c4b-4281-b951-d872f2087c98

chrome/browser/memory_purger.cc

@@ -67,7 +67,9 @@ void PurgeMemoryIOHelper::PurgeMemoryOnIOThread() {
         PurgeMemory();
   }
 
+#if defined(ENABLE_SAFE_BROWSING)
   safe_browsing_service_->PurgeMemory();
+#endif
 }
 
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