Remove about:flags#enable-spdy4 flag.

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index 1f7ad6f405204985db0851d3ca630e044b32d33f..05f6aa776fcd4b421c365d5e1d00627ef00a4c50 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -56343,7 +56343,6 @@ To add a new entry, add it with any value and run test to compute valid value.
   <int value="368854020" label="ash-screen-rotation-animation"/>
   <int value="370486304" label="enable-origin-chip-on-srp"/>
   <int value="400322063" label="ash-disable-screen-orientation-lock"/>
-  <int value="401983950" label="enable-spdy4"/>

[Alexei Svitkine (slow), 2015/05/11 21:01:59]

Don't remove this, as it's still needed to decode data from old versions.

   <int value="402143634" label="enable-search-button-in-omnibox-always"/>
   <int value="412957264" label="tab-close-buttons-hidden-with-touch"/>
   <int value="413081240" label="enable-new-md-input-view"/>