Remove "non-standard" from some event IDLs

srcElement and returnValue were specified in [1] and have WPTs in [2].

cancelBubble is documented in [3] and has a WPT which passes in all
browsers in [4].

[1] https://github.com/whatwg/dom/pull/407
[2] https://github.com/web-platform-tests/wpt/pull/10258
[3] https://developer.mozilla.org/en-US/docs/Web/API/Event/cancelBubble
[4] https://wpt.fyi/results/dom/events/Event-cancelBubble.html

Fixed: 692695
Change-Id: Ia7113a9996a0e272829a3e81bf751412444e9ba8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2610947Reviewed-by: Mason Freed <masonfreed@chromium.org>
Commit-Queue: Joey Arhar <jarhar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840397}

third_party/blink/renderer/core/dom/events/event.idl

@@ -51,9 +51,10 @@
 
     [Measure] void initEvent(DOMString type, optional boolean bubbles = false, optional boolean cancelable = false);
 
-    // Non-standard APIs
     [MeasureAs=EventSrcElement] readonly attribute EventTarget srcElement;
     [MeasureAs=EventReturnValue, CallWith=ScriptState, ImplementedAs=legacyReturnValue] attribute boolean returnValue;
     [MeasureAs=EventCancelBubble, CallWith=ScriptState] attribute boolean cancelBubble;
+
+    // Non-standard APIs
     [MeasureAs=EventPath, CallWith=ScriptState] readonly attribute object path;
 };