In inspector code everywhere except V8StackTraceImpl we get pointer to isolate from inspector object or v8::FunctionCallbackInfo. We can avoid usage of v8::Isolate::GetCurrent in V8StackTraceImpl too. It will simplify a little embedder code by removing requirement to have v8::Isolate::Scope before calling to V8InspectorSession::dispatchProtocolMessage.

BUG=v8:5907
R=dgozman@chromium.org

Review-Url: https://codereview.chromium.org/2789593002
Cr-Commit-Position: refs/heads/master@{#44282}