[webui][ntp] Split theme icons fill into two equally wide semi circles

Before, the left (almost) semi circle was a bit narrower than the right
one. This regressed because the semi circle width was expressed in
absolute units and didn't adapt to changes of the overall theme icon
size. This CL changes the semi circle width to percentages and thus
makes it robust to future theme icon size changes.

Fixed: 1070588
Change-Id: I5eab8a2ebce1646a5f4f54e2c3191c3b3751c757
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2152773
Auto-Submit: Tibor Goldschwendt <tiborg@chromium.org>
Commit-Queue: Moe Ahmadi <mahmadi@chromium.org>
Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759753}

chrome/browser/resources/new_tab_page/theme_icon.html

@@ -45,8 +45,8 @@
 <svg xmlns="http://www.w3.org/2000/svg"
     xmlns:xlink="http://www.w3.org/1999/xlink">
   <defs>
-    <linearGradient id="gradient" gradientUnits="userSpaceOnUse" x1="32" y1="32"
-        x2="32.08" y2="32">
+    <linearGradient id="gradient" gradientUnits="objectBoundingBox"
+        x1="50%" y1="0" x2="50.01%" y2="0">
       <stop id="leftColor" offset="0%"></stop>
       <stop id="rightColor" offset="100%"></stop>
     </linearGradient>