[tracing] Clean up the MemoryInfra API surface (2/3)
This patch simplifies the API surface of MemoryInfra switching
attribute names from std::string to char*.
The main goal is to make the dumping fast and predictable memory-wise.
There is no point allowing the attribute names to be arbitrary strings
(read non static "quoted" char* ptrs). That has the only effect of
causing loads of copies and std::string ctors / dtors.
BUG=495628,480500
Review URL: https://codereview.chromium.org/
1180733002
Cr-Commit-Position: refs/heads/master@{#333934}