MemoryInfra: Add missing sync model type MOUNTAIN_SHARE

This new model type was introduced in https://crrev.com/c/1068528, which
failed to add the new whitelist entry. Similar to https://crbug.com/818570.

Bug: 846252
Change-Id: Ie11443d48f62cc034bf177ff0e7d35d06aa6c2c8
Reviewed-on: https://chromium-review.googlesource.com/1070139Reviewed-by: Primiano Tucci <primiano@chromium.org>
Reviewed-by: Siddhartha S <ssid@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561973}

base/trace_event/memory_infra_background_whitelist.cc

@@ -291,6 +291,7 @@ const char* const kAllocatorDumpNameWhitelist[] = {
     "sync/0x?/model_type/MANAGED_USER_SETTING",
     "sync/0x?/model_type/MANAGED_USER_SHARED_SETTING",
     "sync/0x?/model_type/MANAGED_USER_WHITELIST",
+    "sync/0x?/model_type/MOUNTAIN_SHARE",
     "sync/0x?/model_type/NIGORI",
     "sync/0x?/model_type/PASSWORD",
     "sync/0x?/model_type/PREFERENCE",