Avoid div-by-zero in thumb quad computation

cc/layers/scrollbar_layer_unittest.cc

Index: cc/layers/scrollbar_layer_unittest.cc
diff --git a/cc/layers/scrollbar_layer_unittest.cc b/cc/layers/scrollbar_layer_unittest.cc
index 7b5eb9eac8fe5fab6152e0db21f114e932e593fe..bcec55ac145bf6bd931ec92dd28e9574efc8c168 100644
--- a/cc/layers/scrollbar_layer_unittest.cc
+++ b/cc/layers/scrollbar_layer_unittest.cc
@@ -274,7 +274,7 @@ TEST(ScrollbarLayerTest, ThumbRect) {
 
 TEST(ScrollbarLayerTest, SolidColorDrawQuads) {
   const int kThumbThickness = 3;
-  const int kTrackStart = 0;
+  const int kTrackStart = 1;

[weiliangc, 2014/07/22 18:49:46]

I don't understand why we need to change TrackStart constant in test?

[Ian Vollick, 2014/07/22 18:55:00]

It's a good question. I did this so that I can more reliably check that we set
the TrackStart value when we avoid the if block in scrollbar_layer_base_impl
when maximum_ is zero (rather than coincidentally winding up with zero if the
numerics fell apart).

I also think it's a nicer value than zero, even for the other tests -- if it's
non-zero, we can actually see it being used.

[weiliangc, 2014/07/22 19:00:12]

Ah didn't realize the first test case has 40->39 and second test case 80->78 is
because it's 2x the first time, thought second case decremented more than first
test case because of rounding issue.

Yeah, looks good. 

   const int kTrackLength = 100;
 
   LayerTreeSettings layer_tree_settings;
@@ -303,7 +303,7 @@ TEST(ScrollbarLayerTest, SolidColorDrawQuads) {
     const QuadList& quads = render_pass->quad_list;
     ASSERT_EQ(1u, quads.size());
     EXPECT_EQ(DrawQuad::SOLID_COLOR, quads[0]->material);
-    EXPECT_RECT_EQ(gfx::Rect(6, 0, 40, 3), quads[0]->rect);
+    EXPECT_RECT_EQ(gfx::Rect(6, 0, 39, 3), quads[0]->rect);
   }
 
   // Contents scale should scale the draw quad.
@@ -319,7 +319,7 @@ TEST(ScrollbarLayerTest, SolidColorDrawQuads) {
     const QuadList& quads = render_pass->quad_list;
     ASSERT_EQ(1u, quads.size());
     EXPECT_EQ(DrawQuad::SOLID_COLOR, quads[0]->material);
-    EXPECT_RECT_EQ(gfx::Rect(12, 0, 80, 6), quads[0]->rect);
+    EXPECT_RECT_EQ(gfx::Rect(12, 0, 78, 6), quads[0]->rect);
   }
   scrollbar_layer_impl->draw_properties().contents_scale_x = 1.f;
   scrollbar_layer_impl->draw_properties().contents_scale_y = 1.f;
@@ -337,7 +337,23 @@ TEST(ScrollbarLayerTest, SolidColorDrawQuads) {
     const QuadList& quads = render_pass->quad_list;
     ASSERT_EQ(1u, quads.size());
     EXPECT_EQ(DrawQuad::SOLID_COLOR, quads[0]->material);
-    EXPECT_RECT_EQ(gfx::Rect(8, 0, 20, 3), quads[0]->rect);
+    EXPECT_RECT_EQ(gfx::Rect(8, 0, 19, 3), quads[0]->rect);
+  }
+
+  // We shouldn't attempt div-by-zero when the maximum is zero.
+  scrollbar_layer_impl->SetCurrentPos(0.f);
+  scrollbar_layer_impl->SetMaximum(0);
+  {
+    MockOcclusionTracker<LayerImpl> occlusion_tracker;
+    scoped_ptr<RenderPass> render_pass = RenderPass::Create();
+    AppendQuadsData data;
+    scrollbar_layer_impl->AppendQuads(
+        render_pass.get(), occlusion_tracker, &data);
+
+    const QuadList& quads = render_pass->quad_list;
+    ASSERT_EQ(1u, quads.size());
+    EXPECT_EQ(DrawQuad::SOLID_COLOR, quads[0]->material);
+    EXPECT_RECT_EQ(gfx::Rect(1, 0, 19, 3), quads[0]->rect);
   }
 }