Optimize machine code size of UMA histogram macros.

Optimize machine code size of UMA histogram macros.

Adds overloads to the histogram FactoryGet() functions
that take a const char* param, instead of std::string for
the name. This avoids the std::string constructor being
inlined at call-sites when a C string is passed (but call
sites that pass std::strings already will continue to use
the std::string functions).

In my Mac Release build, this change results in a 271K
reduction to Chromium Framework binary size.

BUG=343946

Review URL: https://codereview.chromium.org/1195473002

Cr-Commit-Position: refs/heads/master@{#335178}