GitLab

This CL optimizes TracedValue::AppendAsTraceFormat() by removing
intermediate base::Value conversion.

HeapProfilerPerfTest.AppendStackFramesAsTraceFormat shows ~1.9x improvement
on macOS (1730ms -> 920ms).

TBR=jbauman@chromium.org

BUG=739378, 664350

Review-Url: https://codereview.chromium.org/2975033002
Cr-Commit-Position: refs/heads/master@{#487019}