Add heuristics to skip thumbnail generation when it's unnecessary.

chrome/common/thumbnail_score_unittest.cc

 // Copyright (c) 2009 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.
 
 #include "chrome/common/thumbnail_score.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 // Tests that the different types of thumbnails are compared properly.
 TEST(ThumbnailScoreTest, ShouldReplaceThumbnailWithType) {
   base::Time now = base::Time::Now();
@@ skipping 34 matching lines @@
   some_redirects.redirect_hops_from_dest = 1;
 
   EXPECT_TRUE(ShouldReplaceThumbnailWith(some_redirects, no_redirects));
 
   // This one has a lot of redirects but a better score. It should still be
   // rejected.
   ThumbnailScore lotsa_redirects(0.4, true, true, now);
   lotsa_redirects.redirect_hops_from_dest = 4;
   EXPECT_FALSE(ShouldReplaceThumbnailWith(no_redirects, lotsa_redirects));
 }
+
+TEST(ThumbnailScoreTest, ShouldConsiderUpdating) {
+  ThumbnailScore score;
+  // By default, the score is 1.0, meaning very boring, thus we should
+  // generate a new thumbnail.
+  EXPECT_DOUBLE_EQ(1.0, score.boring_score);
+  EXPECT_TRUE(score.ShouldConsiderUpdating());
+
+  // Make it very interesting, but this is not enough.
+  score.boring_score = 0.0;
+  EXPECT_TRUE(score.ShouldConsiderUpdating());
+
+  // good_clipping is important, but sill not enough.
+  score.good_clipping = true;
+  EXPECT_TRUE(score.ShouldConsiderUpdating());
+
+  // at_top is important. Finally, the thumbnail is new and interesting enough.
+  score.at_top = true;
+  EXPECT_FALSE(score.ShouldConsiderUpdating());
+
+  // Make it old. Then, it's no longer new enough.
+  score.time_at_snapshot -= ThumbnailScore::kUpdateThumbnailTime;
+  EXPECT_TRUE(score.ShouldConsiderUpdating());
+}