Add components/gc/ to blink_heap watchlist

TBR=haraken@chromium.org

Change-Id: Iaf663f0ea1fcb4a1c2870846d97fbe9580c5afdb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2079725Reviewed-by: Keishi Hattori <keishi@chromium.org>
Commit-Queue: Keishi Hattori <keishi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745398}

WATCHLISTS

@@ -406,7 +406,8 @@
     },
     'blink_heap': {
       'filepath': 'third_party/blink/renderer/platform/heap' \
-                  '|base/allocator/partition_allocator/'
+                  '|base/allocator/partition_allocator/' \
+                  '|components/gc/'
     },
     'blink_html': {
       'filepath': 'third_party/blink/renderer/core/html/' \