Update beta/stable merge process to include discussing edge cases with TPM

The current merge process for Phase 3 (late beta/stable) doesn't cover
cases where it's unclear whether the severity of the issue meets the
requirements for merging. This CL adds a segment to tell the engineer to
consult the TPM/their manager.

Bug: 1015723
Change-Id: Ia8406f47be7e0ed6ce87b6f4b3bf4cc9ecca78ad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1868213Reviewed-by: Josafat Garcia <josafat@chromium.org>
Commit-Queue: Alan Cutter <alancutter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707265}

docs/process/merge_request.md

@@ -86,6 +86,10 @@ fix is low complexity.
 Security bugs should be consulted with [chrome-security@](chrome-security@google.com)
 to determine criticality.
 
+If it is unclear whether the severity of the issue meets the bar for merging
+consult with the [TPM](https://chromiumdash.appspot.com/schedule) and your
+manager.
+
 This table below provides key dates and phases as an example, for M61 release.
 
 Key Event  | Date