Removed use of invalid V8 execution context.

IDBRequest::stop() (override of ActiveDOMObject's) calls
DOMRequestState::clear() which sets DOMRequestState::m_executionContext to
NULL.  ::idbAnyToScriptValue assumes state (an instance of DOMRequestState)
will never return NULL when calling DOMRequestState::context()

Later on IDBRequest::result() is calling idbAnyToScriptValue() with the
stopped IDBRequest::m_requestState.

Also ::scriptValueToIDBKey (and all other functions in IDBBindingUtilities.cpp)
use state->context()->GetIsolate() to get the isolate. The isolate still exists,
but the context is now NULL. Changing to state->isolate() fixes this problem.

BUG=345354

Review URL: https://codereview.chromium.org/180733005

git-svn-id: svn://svn.chromium.org/blink/trunk@169032 bbb929c8-8fbe-4397-9dbb-9b2b20218538