Add IPv6 probing support, and disable IPv6 resolution when it is useless

I've added minimal probing to check if IPv6 is at all possible, and when it is not, then we disable IPv6 resolution. 

I've also added histograms and A/B test support to evaluate the impact of 
this change. 

(I landed originally, but had tree problems, and this is a new CL to tryto reland).

Note that I've switched back to MACRO style enums as well, per http://dev.chromium.org/developers/coding-style
(search for "enum").

This version now does the conditional testing at a higher level (in io_thread.h), so that it should interfere less with other testing.

r=wtc,eroman 
Review URL: http://codereview.chromium.org/585005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@38402 0039d316-1c4b-4281-b951-d872f2087c98

2 files changed, 8 insertions, 0 deletions

diff --git a/chrome/common/chrome_switches.cc b/chrome/common/chrome_switches.cc
index 5d76c4d..d3fdbcb 100644
--- a/chrome/common/chrome_switches.cc
+++ b/chrome/common/chrome_switches.cc
@@ -104,6 +104,7 @@ const char kDisableHangMonitor[]            = "disable-hang-monitor";
 // Don't resolve hostnames to IPv6 addresses. This can be used when debugging
 // issues relating to IPv6, but shouldn't otherwise be needed. Be sure to
 // file bugs if something isn't working properly in the presence of IPv6.
+// This flag can be overidden by the "enable-ipv6" flag.
 const char kDisableIPv6[]                   = "disable-ipv6";
 
 // Prevent images from loading.
@@ -221,6 +222,12 @@ const char kEnableGeolocation[]             = "enable-geolocation";
 // Enable the Indexed Database API.
 const char kEnableIndexedDatabase[]         = "enable-indexed-database";
 
+// Enable IPv6 support, even if probes suggest that it may not be fully
+// supported.  Some probes may require internet connections, and this flag will
+// allow support independent of application testing.
+// This flag overrides "disable-ipv6" which appears elswhere in this file.
+const char kEnableIPv6[]                    = "enable-ipv6";
+
 // Enable the GPU plugin and Pepper 3D rendering.
 const char kEnableGPUPlugin[]               = "enable-gpu-plugin";
 
diff --git a/chrome/common/chrome_switches.h b/chrome/common/chrome_switches.h
index f7c86aa..e6be99f 100644
--- a/chrome/common/chrome_switches.h
+++ b/chrome/common/chrome_switches.h
@@ -80,6 +80,7 @@ extern const char kEnableFileCookies[];
 extern const char kEnableGeolocation[];
 extern const char kEnableGPUPlugin[];
 extern const char kEnableIndexedDatabase[];
+extern const char kEnableIPv6[];
 extern const char kEnableLogging[];
 extern const char kEnableMonitorProfile[];
 extern const char kEnableNativeWebWorkers[];