[Windows] Make KeyEvent is-repeat flag consistent with other platforms.

In https://chromium-review.googlesource.com/c/chromium/src/+/754130 we
switched to populating the is-repeat flag of ui::KeyEvent solely based
on the flags in the underlying native event, if present.

In the case of a key event triggering a pop-up window to appear, this
caused the first auto-repeat of that event to be dispatched without the
repeat flag set.

To address this we restore the "fallback" path to manually determine
whether an event should be flagged as a repeat, in the case of where a
native event does not itself have the repeat flag set.

Bug: 895266
Change-Id: Ia224e38768d67bf0dc74d60d7e5ed7b6b6663d80
Reviewed-on: https://chromium-review.googlesource.com/c/1281874Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600116}

ui/events/event.cc

@@ -1083,10 +1083,9 @@ bool KeyEvent::IsRepeated(const KeyEvent& event) {
     // Bit 30 of lParam represents the "previous key state". If set, the key
     // was already down, therefore this is an auto-repeat.
     is_repeat = (event.native_event().lParam & 0x40000000) != 0;
-  } else
+  }
 #endif
-  {
-    // Note that this is only reach for non-native events under Windows.
+  if (!is_repeat) {
     if (event.key_code() == (*last_key_event)->key_code() &&
         event.flags() == ((*last_key_event)->flags() & ~ui::EF_IS_REPEAT) &&
         (event.time_stamp() - (*last_key_event)->time_stamp())