More complex content in testWebViewClipWithCircle

Rendering a pure solid color in webview skips over some rendering
infrastructure inside webview, which then hides the issue in
b/160958008. This changes webview to render a blue box for the left 50%
of the content and a red box for the right 50%. With this change, this
test fails where b/160958008 is not fixed and passes when it is fixed,
so serves as a regression test.

Test: testWebViewClipWithCircle passes when bug is fixed and fails when
bug is not fixed.
Bug: 161926977

Change-Id: I9100fdd56296aa6e5e324bfc275e40ce07f89989