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I remembered some RAPPOR tasks being Janky and looked
at sampling profiler data for this code and noticed there
was a lot of string copies. This CL makes some intermediate
parts use StringPiece to avoid some copies. There's still
more copies to go, but this should reduce how many we need.

BUG=none

Review-Url: https://codereview.chromium.org/2156383002
Cr-Commit-Position: refs/heads/master@{#406125}