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}
void WebKitTestController::OnOverridePreferences(const WebPreferences& prefs) {
  should_override_prefs_ = true;
  prefs_ = prefs;
}

void WebKitTestController::OnOverridePreferences(const WebPreferences& prefs) {
  should_override_prefs_ = true;
  prefs_ = prefs;
}
