Remove linux_use_tcmalloc and switch to use_allocator in Blink.

See https://src.chromium.org/viewvc/blink?revision=168591&view=revision,
the bug and http://crrev.com/255129 for the details.

BUG=345554

Review URL: https://codereview.chromium.org/259543003

git-svn-id: svn://svn.chromium.org/blink/trunk@172528 bbb929c8-8fbe-4397-9dbb-9b2b20218538