content: Rename webkit_test_helpers.{cc,h} to blink_test_helpers.{cc,h}

Now that webkit/ is gone, we are preparing ourselves for the merge of
third_party/WebKit into //blink.

BUG=None
BUG=content_shell && content_unittests
R=avi@chromium.org

Review URL: https://codereview.chromium.org/1118183003

Cr-Commit-Position: refs/heads/master@{#328202}
WebKitTestResultPrinter::WebKitTestResultPrinter(
    std::ostream* output, std::ostream* error)
    : state_(DURING_TEST),
      capture_text_only_(false),
      encode_binary_data_(false),
      output_(output),
      error_(error) {
}

WebKitTestResultPrinter::WebKitTestResultPrinter(
    std::ostream* output, std::ostream* error)
    : state_(DURING_TEST),
      capture_text_only_(false),
      encode_binary_data_(false),
      output_(output),
      error_(error) {
}
