Fix matrix interpolation between rotations of 180 degrees

ui/gfx/geometry/quaternion_unittest.cc

Index: ui/gfx/geometry/quaternion_unittest.cc
diff --git a/ui/gfx/geometry/quaternion_unittest.cc b/ui/gfx/geometry/quaternion_unittest.cc
index ef460151d37bbd1f0fe2f25234a2ced6aec97b71..cf9ea352bc0cb598d81d722530dd608f80939f0b 100644
--- a/ui/gfx/geometry/quaternion_unittest.cc
+++ b/ui/gfx/geometry/quaternion_unittest.cc
@@ -142,16 +142,15 @@ TEST(QuatTest, SlerpOppositeAngles) {
   Quaternion start(axis, start_radians);
   Quaternion stop(axis, stop_radians);
 
-  // When quaternions are pointed in the fully opposite direction, we take the
-  // interpolated quaternion to be the first. This is arbitrary, but if we
-  // change this policy, this test should fail.
-  Quaternion expected = start;
-
-  for (size_t i = 0; i < 100; ++i) {
-    float t = static_cast<float>(i) / 100.0f;
-    Quaternion interpolated = start.Slerp(stop, t);
-    CompareQuaternions(expected, interpolated);
-  }
+  // When quaternions are pointed in the fully opposite direction, we rotate in
+  // an arbitrary direction.

[danakj, 2017/07/06 15:41:06]

Do you not want to mention/link to spec stuff?

[Ian Vollick, 2017/07/06 15:45:59]

Done.

+  Quaternion expected(axis, 0);
+
+  Quaternion interpolated = start.Slerp(stop, 0.5f);
+  EXPECT_NEAR(expected.x(), interpolated.x(), kEpsilon);
+  EXPECT_NEAR(expected.y(), interpolated.y(), kEpsilon);
+  EXPECT_NEAR(expected.z(), interpolated.z(), kEpsilon);
+  EXPECT_NEAR(expected.w(), interpolated.w(), kEpsilon);
 }
 
 }  // namespace gfx