runtime: clear js idle timeout before new one and after event handler

Updates #6239
Updates #27707

Change-Id: I0a62c1374db485dd830bf02e59625997d9247fc3
Reviewed-on: https://go-review.googlesource.com/c/go/+/203890
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/lock_js.go

@@ -158,6 +158,7 @@ var idleID int32
 // If an event handler returned, we resume it and it will pause the execution.
 func beforeIdle(delay int64) bool {
 	if delay > 0 {
+		clearIdleID()
 		if delay < 1e6 {
 			delay = 1
 		} else if delay < 1e15 {
@@ -229,6 +230,7 @@ func handleEvent() {
 func handleAsyncEvent() {
 	isHandlingEvent = true
 	eventHandler()
+	clearIdleID()
 	isHandlingEvent = false
 }