Reset |_contextMenuNeedsDisplay| when context menu is displayed.

This boolean needs to be reset in order to prevent an extra |Cancel| hit
for the ContextMenu.DelayedElementDetails metric.

Bug: None
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I271a8d4ca58b854b92c1b09d8a8346f0f00d65a4
Reviewed-on: https://chromium-review.googlesource.com/1159600Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Mike Dougherty <michaeldo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580269}

ios/web/web_state/ui/crw_context_menu_controller.mm

@@ -383,6 +383,7 @@ struct ContextMenuInfo {
 - (void)setDOMElementForLastTouch:(NSDictionary*)element {
   _contextMenuInfoForLastTouch.dom_element = [element copy];
   if (_contextMenuNeedsDisplay) {
+    _contextMenuNeedsDisplay = NO;
     UMA_HISTOGRAM_ENUMERATION(kContextMenuDelayedElementDetailsHistogram,
                               DelayedElementDetailsState::Show);
     [self processReceivedDOMElement];