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This CL instruments SdchManager using MemoryDumpProvider.
Even though SDCH might be unshipped (see blink-dev),
it is useful to get an idea of how much memory dictionaries
cost.

BUG=669108

Review-Url: https://codereview.chromium.org/2541073003
Cr-Commit-Position: refs/heads/master@{#441485}