Cleanup transform-related methods in RenderLayer.

-Get rid of paintsWithTransform.
-Remove currentTransform. It was only ever called with
IncludeTransformOrigin and on non-null transforms.
-Remove renderableTransform. It was only called on
non-null transforms.
-Remove a layer()->parent() branch. The ASSERT had been
added in a previous patch, but the branch wasn't removed.
-Inline makeMatrixRenderable. It was only called in one
place.
-Add a test to make sure that transform origin is corrrectly
excluded from the transform computedStyle.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/958463002

engine/src/flutter/tests/styles/transforms-expected.txt

@@ -0,0 +1,8 @@
+CONSOLE: unittest-suite-wait-for-done
+CONSOLE: PASS: pixel translate should roundtrip
+CONSOLE: PASS: percent translate should roundtrip
+CONSOLE: PASS: scale should roundtrip
+CONSOLE: 
+CONSOLE: All 3 tests passed.
+CONSOLE: unittest-suite-success
+DONE

engine/src/flutter/tests/styles/transforms.sky

@@ -0,0 +1,52 @@
+<style>
+  .container {
+    height: 100px;
+    width: 200px;
+    margin: 30px;
+    outline: 1px solid black;
+  }
+  .box {
+    height: 100%;
+    width: 100%;
+    padding: 5px;
+    margin: 5px;
+    border: 5px solid gray;
+    background-color: green;
+    transform-origin: 20% 50%;
+  }
+</style>
+
+<div class="container">
+  <div id="test-box" class="box"></div>
+</div>
+
+<script type="text/javascript" charset="utf-8">
+  import "../resources/third_party/unittest/unittest.dart";
+  import "../resources/unit.dart";
+
+  import "dart:sky";
+
+  void main() {
+    initUnit();
+
+    var testBox = document.getElementById('test-box');
+
+    void testTransformRoundTrip(transform, resultMatrix) {
+      testBox.style['transform'] = transform;
+      var computedTransform = window.getComputedStyle(testBox)['transform'];
+      expect(computedTransform, equals(resultMatrix));
+    }
+
+    test('pixel translate should roundtrip', () {
+      testTransformRoundTrip('translate(80px, 90px)', 'matrix(1, 0, 0, 1, 80, 90)');
+    });
+
+    test('percent translate should roundtrip', () {
+      testTransformRoundTrip('translate(10px, 50%)', 'matrix(1, 0, 0, 1, 10, 60)');
+    });
+
+    test('scale should roundtrip', () {
+      testTransformRoundTrip('scale(1.2, 0.8)', 'matrix(1.2, 0, 0, 0.8, 0, 0)');
+    });
+  }
+</script>