Don't calculate a shown ratio when top controls height is 0.

This was causing a crash in Blink with some Chrome Android UI pages. These
pages would disable top controls by setting the height to 0 but when we
scroll the page we calculate the "shown ratio" by dividing by height. This
the divided-by-0 value made its way down to Blink.

BUG=460007

Review URL: https://codereview.chromium.org/982333003

Cr-Commit-Position: refs/heads/master@{#319621}

cc/input/top_controls_manager.cc

@@ -104,6 +104,9 @@ gfx::Vector2dF TopControlsManager::ScrollBy(
   if (pinch_gesture_active_)
     return pending_delta;
 
+  if (!TopControlsHeight())
+    return pending_delta;
+
   if (permitted_state_ == SHOWN && pending_delta.y() > 0)
     return pending_delta;
   else if (permitted_state_ == HIDDEN && pending_delta.y() < 0)

cc/input/top_controls_manager_unittest.cc

@@ -5,7 +5,9 @@
 #include "cc/input/top_controls_manager.h"
 
 #include <algorithm>
+#include <cmath>
 
+#include "base/logging.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/time/time.h"
 #include "cc/input/top_controls_manager_client.h"
@@ -51,6 +53,9 @@ class MockTopControlsManagerClient : public TopControlsManagerClient {
   float TopControlsHeight() const override { return top_controls_height_; }
 
   void SetCurrentTopControlsShownRatio(float ratio) override {
+    ASSERT_FALSE(std::isnan(ratio));
+    ASSERT_FALSE(ratio == std::numeric_limits<float>::infinity());
+    ASSERT_FALSE(ratio == -std::numeric_limits<float>::infinity());
     ratio = std::max(ratio, 0.f);
     ratio = std::min(ratio, 1.f);
     top_controls_shown_ratio_ = ratio;
@@ -445,5 +450,20 @@ TEST(TopControlsManagerTest, ShrinkingHeightKeepsTopControlsHidden) {
   EXPECT_FLOAT_EQ(0.f, manager->ContentTopOffset());
 }
 
+TEST(TopControlsManagerTest, ScrollByWithZeroHeightControlsIsNoop) {
+  MockTopControlsManagerClient client(0.f, 0.5f, 0.5f);
+  TopControlsManager* manager = client.manager();
+  manager->UpdateTopControlsState(BOTH, BOTH, false);
+
+  manager->ScrollBegin();
+  gfx::Vector2dF pending = manager->ScrollBy(gfx::Vector2dF(0.f, 20.f));
+  EXPECT_FLOAT_EQ(20.f, pending.y());
+  EXPECT_FLOAT_EQ(0.f, manager->ControlsTopOffset());
+  EXPECT_FLOAT_EQ(0.f, manager->ContentTopOffset());
+  EXPECT_FLOAT_EQ(1.f, client.CurrentTopControlsShownRatio());
+  manager->ScrollEnd();
+}
+
+
 }  // namespace
 }  // namespace cc