DevTools: Prevent the ARIA alert from obscuring DevTools

The alert element must be visible for a screen reader to read it,
so we place it far off the left edge of the screen. However extremely
large text could cause it to overflow onto the page. This patch
limits the width of the element, and also limits the max text length
of the alerts for better performance.

Bug: 845766
Change-Id: I79aadcf863f09ebfdaf2bcb3ba579b891fec2351
Reviewed-on: https://chromium-review.googlesource.com/1067690
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Joel Einbinder <einbinder@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561490}

third_party/blink/renderer/devtools/front_end/ui/ARIAUtils.js

@@ -149,11 +149,13 @@ UI.ARIAUtils.alert = function(message, element) {
     const alertElement = document.body.createChild('div');
     alertElement.style.position = 'absolute';
     alertElement.style.left = '-999em';
+    alertElement.style.width = '100em';
+    alertElement.style.overflow = 'hidden';
     alertElement.setAttribute('role', 'alert');
     alertElement.setAttribute('aria-atomic', 'true');
     document[UI.ARIAUtils.AlertElementSymbol] = alertElement;
   }
-  document[UI.ARIAUtils.AlertElementSymbol].textContent = message;
+  document[UI.ARIAUtils.AlertElementSymbol].textContent = message.trimEnd(10000);
 };
 
 UI.ARIAUtils.AlertElementSymbol = Symbol('AlertElementSybmol');