Rename WebTestProxy to WebViewTestProxy.

As we continue to split WebView/WebWidget, this change renames
WebTestProxy to WebViewTestProxy, since it should only used for
proxying RenderView calls. A future change will introduce a new
WebWidgetTestProxy for use with RenderWidget.

BUG=583347

Review-Url: https://codereview.chromium.org/2171503005
Cr-Commit-Position: refs/heads/master@{#407165}
35 files changed