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The chrome://tcmalloc page has been superseded by the memory-infra
category of chrome://tracing. The page no longer provides enough value
to justify its complexity in the codebase, so unlaunch it in the light
of Project Eraser.

Furthermore, this allows removing |base::allocator::GetStats|, which is
not used anywhere except in chrome:://tcmalloc. Detailed information
about memory allocators is available in the memory-infra category of
chrome://tracing. Unlike |GetStats|, which was implemented for tcmalloc
only, memory-infra provides detailed information about all allocators.

BUG=560250

Review URL: https://codereview.chromium.org/1468993003

Cr-Commit-Position: refs/heads/master@{#361402}