More general test for TransformBetweenRects

The current test case for gfx::TransformBetweenRects only verifies
the function on the source rectangle whose origin is at (0,0).
This CL provides a more general test case.

Bug: 1046983
Change-Id: I06a451ce7bcbad1344139792d6265d5313fe782d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2036272Reviewed-by: Ian Vollick <vollick@chromium.org>
Commit-Queue: Andrew Xu <andrewxu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#739110}

ui/gfx/transform_util_unittest.cc

@@ -320,13 +320,29 @@ TEST(TransformUtilTest, Transform2D) {
 }
 
 TEST(TransformUtilTest, TransformBetweenRects) {
-  const RectF src_rect(0.f, 0.f, 5.f, 5.f);
-  const RectF dst_rect(10.f, 10.f, 10.f, 20.f);
-
-  Transform transform = TransformBetweenRects(src_rect, dst_rect);
-  RectF transformed_rect = src_rect;
-  transform.TransformRect(&transformed_rect);
-  EXPECT_EQ(dst_rect, transformed_rect);
+  auto verify = [](const RectF& src_rect, const RectF& dst_rect) {
+    const Transform transform = TransformBetweenRects(src_rect, dst_rect);
+
+    // Applies |transform| to calculate the target rectangle from |src_rect|.
+    // Notes that |transform| is in |src_rect|'s local coordinates.
+    RectF dst_in_src_coordinates = RectF(src_rect.size());
+    transform.TransformRect(&dst_in_src_coordinates);
+    RectF dst_in_parent_coordinates = dst_in_src_coordinates;
+    dst_in_parent_coordinates.Offset(src_rect.OffsetFromOrigin());
+
+    // Verifies that the target rectangle is expected.
+    EXPECT_EQ(dst_rect, dst_in_parent_coordinates);
+  };
+
+  std::vector<const std::pair<const RectF, const RectF>> test_cases{
+      {RectF(0.f, 0.f, 2.f, 3.f), RectF(3.f, 5.f, 4.f, 9.f)},
+      {RectF(10.f, 7.f, 2.f, 6.f), RectF(4.f, 2.f, 1.f, 12.f)},
+      {RectF(0.f, 0.f, 3.f, 5.f), RectF(0.f, 0.f, 6.f, 2.5f)}};
+
+  for (const auto& test_case : test_cases) {
+    verify(test_case.first, test_case.second);
+    verify(test_case.second, test_case.first);
+  }
 }
 
 }  // namespace