Double size of persistent-histogram memory for browser.
Double size of persistent-histogram memory for browser.

This shouldn't affect the actual working-set as histograms
will just move from the heap to this memory segment and any
unused portion should just remain "virtual".

Also change the passed name from std::string to char[] since
it's now being passed by StringPiece.

Also change the id of the allocator to a hash of the new
name (without "Allocator" at the end).  It's not yet used
anywhere else.

BUG=546019
TBR=asvitkine

Review URL: https://codereview.chromium.org/1701603002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#375402}

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