Fix border radius on composited children.

third_party/WebKit/LayoutTests/compositing/overflow/border-radius-styles-with-composited-child.html

+<style>
+
+.outer {
+    border: 4px solid black;
+    width: 100px;
+    height: 100px;
+    border-radius: 40px;
+    background-color: lightblue;
+    overflow: hidden;
+    margin: 10px;
+}
+
+.inner {
+    width: 80px;
+    height: 80px;
+    background-color: green;
+    padding: .5em;
+    will-change: transform;
+}
+
+.dashed {
+    border-style: dashed;
+}
+
+.transparent {
+    border-color: rgba(255,0,0,0.5);
+}
+
+</style>
+
+<div class = "outer">
+    <div class = "inner"></div>
+</div>
+
+<div class = "outer dashed">
+    <div class = "inner"></div>
+</div>
+
+<div class = "outer transparent">
+    <div class = "inner"></div>
+</div>