Add core/OWNERS to OWNERS for runtime_enabled_features.json5.

It is common for CLs to have no other changes to platform/ aside from creating
or updating a feature. platform/OWNERS review is more or less a rubber-stamp
in such cases. This expedites landing such CLs.

Change-Id: If5ae29e7969f1c9647b0310b28a689be1bf28d18
Reviewed-on: https://chromium-review.googlesource.com/909412
Commit-Queue: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Rick Byers <rbyers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543411}

third_party/WebKit/Source/platform/OWNERS

@@ -21,8 +21,10 @@ tkent@chromium.org
 vollick@chromium.org
 wangxianzhu@chromium.org
 
-# In addition to the above, API_OWNERS are also adequate reviewers
-# for adding / changing the status of a feature.
+# Any API or core owner can also approve changes to runtime-enabled features.
+# Please make sure to get a review from someone with expertise on the feature
+# you are changing.
 per-file runtime_enabled_features.json5=file://third_party/WebKit/API_OWNERS
+per-file runtime_enabled_features.json5=file://third_party/WebKit/Source/core/OWNERS
 
 # COMPONENT: Platform