[Documentation] Update merge guidelines for feature work

This makes our merge guidelines more permissive for feature work
which is gated behind a flag.

Change-Id: I1b65711006cf04e754570d55c16a12b51ed782f5
Reviewed-on: https://chromium-review.googlesource.com/958035Reviewed-by: Krishna Govind <govind@chromium.org>
Commit-Queue: Alex Mineer <amineer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543551}

docs/process/merge_request.md

@@ -63,9 +63,13 @@ Chrome TPMs.
 
 **Phase 2: First Four Weeks of Beta Rollout**
 
-During the first four weeks of Beta, merges should only be requested if
-the bug is considered either release blocking or
-considered a high-impact regression.
+During the first four weeks of Beta, merges should only be requested if:
+
+* The bug is considered either release blocking or
+  considered a high-impact regression
+* The merge is related to a feature which (1) is entirely gated behind
+  a flag and (2) does not change user functionality in a substantial way
+  (e.g. minor tweaks and metrics code are OK, workflow changes are not)
 
 Security bugs should be consulted with
 [chrome-security@google.com](chrome-security@google.com) to