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 WebKitTestResultPrinter::PrintAudioHeader() {
  DCHECK_EQ(state_, DURING_TEST);
  if (!capture_text_only_)
    *output_ << "Content-Type: audio/wav\n";
  state_ = IN_AUDIO_BLOCK;
}

void WebKitTestResultPrinter::PrintAudioHeader() {
  DCHECK_EQ(state_, DURING_TEST);
  if (!capture_text_only_)
    *output_ << "Content-Type: audio/wav\n";
  state_ = IN_AUDIO_BLOCK;
}
