Add a basic postMessage test for <webview> in a new test suite.

The plan is to run this test in site isolation bots as well when
  <webview> using OOPIF CL lands.

BUG=330264
Test=None

Review URL: https://codereview.chromium.org/1148903003

Cr-Commit-Position: refs/heads/master@{#331061}
7 files changed