DevTools: remove methods used for native memory instrumentation

The instrumentation was dropped and the code is unused. This patch depends on Chromium-side change: https://codereview.chromium.org/510703002/

BUG=None
TBR=pfeldman

Review URL: https://codereview.chromium.org/513673002

git-svn-id: svn://svn.chromium.org/blink/trunk@180961 bbb929c8-8fbe-4397-9dbb-9b2b20218538