Add leakcanary to DEPS and third_party

third_party/leakcanary/leakcanary-android-build-config/com/squareup/leakcanary/BuildConfig.java

Index: third_party/leakcanary/leakcanary-android-build-config/com/squareup/leakcanary/BuildConfig.java
diff --git a/chrome/browser/ui/webui/engagement/site_engagement.mojom b/third_party/leakcanary/leakcanary-android-build-config/com/squareup/leakcanary/BuildConfig.java
similarity index 55%
copy from chrome/browser/ui/webui/engagement/site_engagement.mojom
copy to third_party/leakcanary/leakcanary-android-build-config/com/squareup/leakcanary/BuildConfig.java
index 0d9cc4a8e56b10b9321342a63a20d071448689c6..4882cad0b8d16cbc56556a8825f2fdbbc8810602 100644
--- a/chrome/browser/ui/webui/engagement/site_engagement.mojom
+++ b/third_party/leakcanary/leakcanary-android-build-config/com/squareup/leakcanary/BuildConfig.java
@@ -1,13 +1,10 @@
 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
+package com.squareup.leakcanary;
 
-struct SiteEngagementInfo {
-  string origin;
-  double score;
-};
-
-interface SiteEngagementUIHandler {
-  GetSiteEngagementInfo() => (array<SiteEngagementInfo> info);
-  SetSiteEngagementScoreForOrigin(string origin, double score);
-};
+// Normally generated by Gradle.
+class BuildConfig {
+    static final String LIBRARY_VERSION = "1.4-beta1 (chromium)";
+    static final String GIT_SHA = "unknown"; // Unlikely to be kept up-to-date, so don't try.
+}