Tracing: Add support for PII whitelisting of individual trace event arguments

R=dsinclair,shatch
BUG=546093

Review URL: https://codereview.chromium.org/1415013003

Cr-Commit-Position: refs/heads/master@{#356690}
size_t GetAllocLength(const char* str) { return str ? strlen(str) + 1 : 0; }

size_t GetAllocLength(const char* str) { return str ? strlen(str) + 1 : 0; }
