DevTools: use single overload to retrieve instrumenting agents for an object

We used to have a bunch of InstrumentingAgentsForFoo() to fetch instrumenting agents
for different types passed as first argument to instrumentation methods. This patch
turns them into overloads of instrumentingObjectsFor(). This would allow us to handle
objects inherited from already supported ones without introduction of additional methods.

BUG=
R=vsevik@chromium.org

Review URL: https://codereview.chromium.org/23519004

git-svn-id: svn://svn.chromium.org/blink/trunk@157220 bbb929c8-8fbe-4397-9dbb-9b2b20218538