Don't put every <img> in the page in a WebVector in AwRenderViewExt::OnDocumentHasImagesRequest.
Don't put every <img> in the page in a WebVector in AwRenderViewExt::OnDocumentHasImagesRequest.

This method was doing:

  blink::WebVector<blink::WebElement> images;
  webview->mainFrame()->document().images(images);
  hasImages = !images.isEmpty();

which meant Blink would walk the entire document putting every <img>
into a Vector, then it would copy that Vector into this WebVector all
so this code could check if the Vector wasn't empty.

Instead just use the NodeList version of getElementsByHTMLTagName,
which we can already just call in this file with GetImgChild. This
only walks until it finds the first <img> and doesn't need additional
storage.

Review URL: https://codereview.chromium.org/1220333011

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#337920}

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