Expand runtime enabled feature plumbing's owners

Currently, runtime_enabled_features.json5, the central registry of Blink
runtime-enabled features, has a relatively large OWNERS alias (to API
owners and Blink core owners). Adding a new runtime-enabled feature
associated with an embedder-side base::Feature often involves adding
boilerplate to WebRuntimeFeatures and
//content/child/runtime_features.cc. Unfortunately, these files
are in far-removed directories with few common owners.

This CL expands those files' per-file owners to include renderer/core's
owners, just as blink/renderer/platform/OWNERS currently does for
runtime_enabled_features.json5. For the case of
content/child/runtime_features.cc, the change also adds all top-level
Blink owners.

This should reduce the number of approvals needed to land a new
runtime-enabled feature. It might also improve the quality of scrutiny
these changes get: the comments accompanying these OWNERS aliases
(copied from the comment next to runtime_enabled_features.json5's OWNERS
alias) require that the reviewers have context about the functionality
being affected by the runtime-enabled feature; currently, it might be
hard to find owners for public/platform, renderer/platform, and
//content/child who have context on one's specific feature.

Change-Id: If420895d0256347ca2831e835ef970f766420ff6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2092905Reviewed-by: Kent Tamura <tkent@chromium.org>
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: David Van Cleve <davidvc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#749667}

content/child/OWNERS

@@ -2,3 +2,10 @@ haraken@chromium.org
 
 per-file browser_exposed_child_interfaces.cc=set noparent
 per-file browser_exposed_child_interfaces.cc=file://ipc/SECURITY_OWNERS
+
+# For boilerplate changes propagating runtime-enabled features only, any Blink or
+# blink/renderer/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_features.cc=file://third_party/blink/OWNERS
+per-file runtime_features.cc=file://third_party/blink/renderer/core/OWNERS

third_party/blink/public/platform/OWNERS

@@ -18,3 +18,8 @@ per-file web_video_frame*=lethalantidote@chromium.org
 per-file web_surface_layer_bridge*=file://media/OWNERS
 per-file web_surface_layer_bridge*=mlamouri@chromium.org
 per-file web_surface_layer_bridge*=lethalantidote@chromium.org
+
+# Any 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 web_runtime_features*=file://third_party/blink/renderer/core/OWNERS

third_party/blink/renderer/platform/exported/OWNERS

 per-file notification_data_conversions*=peter@chromium.org
 per-file web_rtc_*=hbos@chromium.org
+
+# Any 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 web_runtime_features*=file://third_party/blink/renderer/core/OWNERS