DevTools: FlameChart: speed up forEachEntry function.

entryToAnchorBox was converted into internal function
member variables were replaced with context variables.
This change gives us ~300ms on 1m entries.

BUG=302913
R=yurys@chromium.org

Review URL: https://codereview.chromium.org/25692002

git-svn-id: svn://svn.chromium.org/blink/trunk@158707 bbb929c8-8fbe-4397-9dbb-9b2b20218538