Disable sending KeyUpdates by default.

68df3af was intended to find buggy
servers that don't implement TLS 1.3 KeyUpdate correctly, and it didn't
take long to find some (see bug). This change disables sending
KeyUpdates by default until we've figured out those sites.

TBR=davidben@chromium.org

Bug: 923685
Change-Id: I44968fa85b2b92e18579e7d0fbd13b4b93ecffff
Reviewed-on: https://chromium-review.googlesource.com/c/1424419
Reviewed-by: Adam Langley <agl@chromium.org>
Commit-Queue: Adam Langley <agl@chromium.org>
Cr-Commit-Position: refs/heads/master@{#624492}

net/base/features.cc

@@ -26,7 +26,7 @@ const base::Feature kSplitCacheByTopFrameOrigin{
     "SplitCacheByTopFrameOrigin", base::FEATURE_DISABLED_BY_DEFAULT};
 
 const base::Feature kTLS13KeyUpdate{"TLS13KeyUpdate",
-                                    base::FEATURE_ENABLED_BY_DEFAULT};
+                                    base::FEATURE_DISABLED_BY_DEFAULT};
 
 }  // namespace features
 }  // namespace net