Always return false on IsHandlingSendingEvent

This fixes an issue on closing Web contents on headless Mac, since it makes think Web contents that the current view is still handling UI events. See https://cs.chromium.org/chromium/src/content/browser/web_contents/web_contents_view.h?rcl=e0bef062337921727eb9b670b417ebd0b6d86fd6&l=131

BUG=713268

Review-Url: https://codereview.chromium.org/2853473002
Cr-Commit-Position: refs/heads/master@{#468132}

headless/lib/browser/headless_shell_application_mac.mm

@@ -9,9 +9,8 @@
 @implementation HeadlessShellCrApplication
 
 - (BOOL)isHandlingSendEvent {
-  // The CrAppProtocol must return true if [NSApplication sendEvent:] is
-  // currently on the stack. seee |CrAppProtocol| in |MessagePumpMac|.
-  return true;
+  // Since headless mode is non-interactive, always return false.
+  return false;
 }
 
 @end