DevTools: make homegrown Map class to be compliant with ES6 equivalent

The patch synchronizes devtools homegrown Map implementations (Map, StringMap and StringMultimap)
with the interface of ES6 Map class. This will make it possible to
easily move to ES6 maps once our version of closure compiler
supports them.

R=vsevik, pfeldman

Review URL: https://codereview.chromium.org/479203002

git-svn-id: svn://svn.chromium.org/blink/trunk@180561 bbb929c8-8fbe-4397-9dbb-9b2b20218538