Add test for "fully overlapping" radial gradient case

This adds a test for the case where the start and end circles of a
radial gradient fully overlap (while having non-zero radii).

Bug: 933413
Change-Id: Id059cb607f6fc137111d2306dd9c9eef1f55b6bc
Reviewed-on: https://chromium-review.googlesource.com/c/1483023
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
Auto-Submit: Fredrik Söderquist <fs@opera.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634711}

third_party/blink/web_tests/external/wpt/svg/pservers/reftests/radialgradient-fully-overlapping.svg

+<svg xmlns="http://www.w3.org/2000/svg" xmlns:h="http://www.w3.org/1999/xhtml">
+  <title>Radial gradient with overlapping start and end circles</title>
+  <metadata>
+    <h:link rel="help" href="https://svgwg.org/svg2-draft/pservers.html#RadialGradientNotes"/>
+    <h:link rel="match" href="reference/green-100x100.svg"/>
+    <h:meta name="assert" content="If the start circle fully overlaps with the end circle, no gradient is drawn."/>
+  </metadata>
+
+  <rect width="100" height="100" fill="green"/>
+
+  <radialGradient id="fully_overlapping" r="100" fr="100">
+    <stop stop-color="red"/>
+    <stop stop-color="red" offset="1"/>
+  </radialGradient>
+  <rect width="100" height="100" fill="url(#fully_overlapping) orange"/>
+</svg>