Remove nonstandard Headers.size/forEach, add ES6 iterator

BUG=418271

Review URL: https://codereview.chromium.org/610043003

git-svn-id: svn://svn.chromium.org/blink/trunk@182958 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Fix uninitialized member in GraphicsLayerDebugInfo

TBR=vollick,pfeldman
BUG=400547

Review URL: https://codereview.chromium.org/614133002

git-svn-id: svn://svn.chromium.org/blink/trunk@182959 bbb929c8-8fbe-4397-9dbb-9b2b20218538