Fix iframe execCtx race

The events are as follows when an iframe is attached:
1. execCtx for about:blank for new frame is set;
2. execCtx for navigated frame with url is set;
3. original about:blank execCtx is destroyed.

If we only work with the original execCtx, then it will be destroyed by
the 3rd event. To prevent this we overwrite the original one with the
new one in the 2nd event.

What this means is that we do not end up in an infinite loop when
waitForExecutionContext is called.

common/frame.go

@@ -447,10 +447,31 @@ func (f *Frame) setContext(world executionWorld, execCtx frameExecutionContext)
 		panic(err)
 	}
 
+	// There is a race condition when it comes to attaching iframes and the
+	// execution context that apply to these frames. What usually occurs is:
+	//
+	// 1. An exec context for about:blank is first set;
+	// 2. A new set event is received for exec context for the url pointing
+	//    to the actual destination for the iframe;
+	// 3. Finally the execution context for about:blank is destroyed, not
+	//    for the second execution context.
+	//
+	// This is the order of events when iframes are in use on a site, and
+	// so it is safe to nil the original execution context and overwrite it
+	// with the second one.
+	//
+	// The exec context destroyed event will not remove the new exec context
+	// since the ids do not match.
+	//
+	// If we didn't overwrite the first execCtx with the new one, then
+	// waitForExecutionContext could end up waiting indefinitely since all
+	// execCtx were destroyed.
 	if f.executionContexts[world] != nil {
-		f.log.Debugf("Frame:setContext", "fid:%s furl:%q ectxid:%d world:%s, world exists",
+		f.log.Debugf("Frame:setContext", "fid:%s furl:%q ectxid:%d world:%s, overriding existing world",
 			f.ID(), f.URL(), execCtx.ID(), world)
-		return
+
+		f.executionContexts[world] = nil
+		f.documentHandle = nil
 	}
 
 	f.executionContexts[world] = execCtx