Send HeapProfiler.enable only when profiles panel is shown

Otherwise on cross-process navigation we get following console error:

[22766:22766:0127/193003:ERROR:CONSOLE(237)] "Protocol Error: the message HeapProfiler.resetProfiles is for non-existing domain 'HeapProfiler'", source: chrome-devtools://devtools/bundled/InspectorBackend.js (237)

BUG=None
R=alph@chromium.org

Review URL: https://codereview.chromium.org/132903010

git-svn-id: svn://svn.chromium.org/blink/trunk@165858 bbb929c8-8fbe-4397-9dbb-9b2b20218538