shadow of a transformed element prevents clicks

[paulrouget]

Affects: browserhtml (can't click on hamburger icon).

Both with and without webrender.

Clicking on the green div under the shadow doesn't print 42.

<style>
  .green {
    background: green;
    position: absolute;
    top: 45px;
    left: 170px;
    width: 100px;
    height: 50px;
  }
  .red {
    background: red;
    position: absolute;
    top: 20px;
    left: 20px;
    width: 100px;
    height: 100px;
    box-shadow: 100px 0 0 rgba(0,0,0,0.1);
    transform: translateX(1px);
  }
</style>
<div class="green" onclick="console.log(42)"></div>
<div class="red"></div>

[jdm]

cc @glennw @mrobinson