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There could be multiple providers. So, each must create allocator dumps
with distinct names. Refer notes on GetOrCreateAllocatorDump.
Group autocomplete and in_memory_url_index dumps under different child
dumps for easier collection of metrics.

BUG=803822
TBR=mpearson@chromium.org

Change-Id: I872491bb1496572600fc6b6f87d9198a76eea8c7
Reviewed-on: https://chromium-review.googlesource.com/961263
Commit-Queue: Siddhartha S <ssid@chromium.org>
Reviewed-by: Justin Donnelly <jdonnelly@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543017}