feat(Grid): add icon-size prop (#4656)

src/grid/test/__snapshots__/index.spec.js.snap

@@ -1,5 +1,14 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
+exports[`icon-size prop 1`] = `
+<div class="van-grid van-hairline--top">
+  <div class="van-grid-item" style="flex-basis: 25%;">
+    <div class="van-grid-item__content van-grid-item__content--center van-hairline"><i class="van-icon van-icon-success van-grid-item__icon" style="font-size: 10px;">
+        <!----></i></div>
+  </div>
+</div>
+`;
+
 exports[`sqaure and set gutter 1`] = `
 <div class="van-grid" style="padding-left: 10rem;">
   <div class="van-grid-item van-grid-item--square" style="flex-basis: 50%; padding-top: 50%;">

src/grid/test/index.spec.js

@@ -38,3 +38,15 @@ test('sqaure and set gutter', () => {
 
   expect(wrapper).toMatchSnapshot();
 });
+
+test('icon-size prop', () => {
+  const wrapper = mount({
+    template: `
+      <van-grid icon-size="10">
+        <van-grid-item icon="success" />
+      </van-grid>
+    `
+  });
+
+  expect(wrapper).toMatchSnapshot();
+});