[DevTools] not run WorkspaceDiff on files with more then 1024*1024 lines

R=lushnikov@chromium.org

Bug: chromium:763752
Change-Id: I002ac8a40cc6a21949a51d926c1c22ddd8b23fc0
Reviewed-on: https://chromium-review.googlesource.com/728731
Commit-Queue: Andrey Lushnikov <lushnikov@chromium.org>
Reviewed-by: Andrey Lushnikov <lushnikov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#510261}

third_party/WebKit/Source/devtools/front_end/workspace_diff/WorkspaceDiff.js

@@ -223,12 +223,14 @@ WorkspaceDiff.WorkspaceDiff.UISourceCodeDiff = class extends Common.Object {
     var current = this._uiSourceCode.workingCopy();
     if (!current && !this._uiSourceCode.contentLoaded())
       current = await this._uiSourceCode.requestContent();
-    // ------------ ASYNC ------------
+    if (current.length > 1024 * 1024)
+      return null;
     if (this._dispose)
       return null;
 
     var baseline = await this._uiSourceCode.requestOriginalContent();
-    // ------------ ASYNC ------------
+    if (baseline.length > 1024 * 1024)
+      return null;
     if (this._dispose)
       return null;