Add test for viewport units in SVG-in-<img> context

Bug: 650147
Change-Id: I4a158f600acb733477d33ebbd6a504490a5b54a4
Reviewed-on: https://chromium-review.googlesource.com/c/1341836
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609346}

third_party/WebKit/LayoutTests/external/wpt/svg/embedded/image-embedding-svg-with-viewport-units.svg

+<svg xmlns="http://www.w3.org/2000/svg"
+     xmlns:xlink="http://www.w3.org/1999/xlink"
+     xmlns:h="http://www.w3.org/1999/xhtml">
+  <metadata>
+    <title>&lt;image&gt; embedding SVG image with viewport units</title>
+    <h:link rel="match" href="support/green-rect-100x100.svg"/>
+  </metadata>
+  <image xlink:href="data:image/svg+xml,&lt;svg xmlns='http://www.w3.org/2000/svg'&gt;&lt;rect width='50vw' height='50vh' fill='green'/&gt;&lt;/svg&gt;"
+         width="200" height="200"/>
+</svg>

third_party/WebKit/LayoutTests/external/wpt/svg/embedded/support/green-rect-100x100.svg

+<svg xmlns="http://www.w3.org/2000/svg">
+  <rect width="100" height="100" fill="green"/>
+</svg>