Add custom Polymer network icon element

Ths CL does the following:
* Introduces a new network icon custom Polymer element
* Adds a simple paper-button to chrome://network to test Polymer in WebUI
* Uses the new custom element in the chrome://network UI

BUG=455499

Review URL: https://codereview.chromium.org/874283006

Cr-Commit-Position: refs/heads/master@{#318114}
22 files changed