Version: ToT (398943), debug build
OS: MaxOS X El Captain

What steps will reproduce the problem?
(0) Use debug build
(1) Open DevTools on the new tab page
(2) Switch to Timeline
(3) Assure JS Profile is ticked
(4) Reload

Observe the following assertion fail in the console:

#
# Fatal error in ../../v8/src/profiler/cpu-profiler.cc, line 530
# Check failed: !is_profiling_.
#

==== C stack trace ===============================

 1: V8_Fatal
 2: v8::internal::CpuProfiler::CollectSample()
 3: v8::CpuProfiler::SetSamplingInterval(int)
 4: blink::V8ProfilerAgentImpl::setSamplingInterval(blink::protocol::String16*, int)
 5: blink::protocol::Profiler::DispatcherImpl::setSamplingInterval(int, std::__1::unique_ptr<blink::protocol::DictionaryValue, std::__1::default_delete<blink::protocol::DictionaryValue> >, blink::protocol::ErrorSupport*)
 6: blink::protocol::Profiler::DispatcherImpl::dispatch(int, blink::protocol::String16 const&, std::__1::unique_ptr<blink::protocol::DictionaryValue, std::__1::default_delete<blink::protocol::DictionaryValue> >)
 7: blink::protocol::UberDispatcher::dispatch(blink::protocol::String16 const&)
 8: blink::V8InspectorSessionImpl::dispatchProtocolMessage(blink::protocol::String16 const&)
 9: blink::InspectorSession::dispatchProtocolMessage(WTF::String const&, WTF::String const&)
10: blink::WebDevToolsAgentImpl::dispatchMessageFromFrontend(int, WTF::String const&, WTF::String const&)
11: blink::WebDevToolsAgentImpl::dispatchOnInspectorBackend(int, int, blink::WebString const&, blink::WebString const&)
12: blink::WebEmbeddedWorkerImpl::dispatchDevToolsMessage(int, int, blink::WebString const&, blink::WebString const&)