Tweak cover blending in wpt/svg/render/reftests/blending-00[12].svg

Set 'mix-blend-mode' to 'normal' on the stroked rects that make up the
"edge cover". Hopefully this avoids aliasing artifacts on the edge cover
itself.

In blending-002.svg, localize the use of (non-auto) isolation to the
content of the test.

BUG=866928

Change-Id: I269e01a4d814f7fd8e202ffdf71c1a1d5dc48347
Reviewed-on: https://chromium-review.googlesource.com/1175825Reviewed-by: Stephen Chenney <schenney@chromium.org>
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Cr-Commit-Position: refs/heads/master@{#583277}

third_party/WebKit/LayoutTests/external/wpt/svg/render/reftests/blending-001.svg

@@ -18,6 +18,10 @@
       mix-blend-mode: screen;
     }
 
+    #edgecover > rect {
+      mix-blend-mode: normal;
+    }
+
     g {
       isolation: isolate;
     }
@@ -32,7 +36,7 @@
     </g>
 
     <!-- Stroke to prevent aliasing from effecting results. -->
-    <g style="mix-blend-mode:normal;fill:none;stroke:black;stroke-width:2px">
+    <g style="fill:none;stroke:black;stroke-width:2px" id="edgecover">
       <rect x="120" y="80"  width="160" height="160"/>
       <rect x="200" y="80"  width="160" height="160"/>
       <rect x="160" y="160" width="160" height="160"/>

third_party/WebKit/LayoutTests/external/wpt/svg/render/reftests/blending-002.svg

@@ -18,21 +18,23 @@
       mix-blend-mode: screen;
     }
 
-    g {
-      isolation: isolate;
+    #edgecover > rect {
+      mix-blend-mode: normal;
     }
   </style>
 
   <g id="test-body-content">
 
-    <rect x="120" y="80"  width="160" height="160" fill="red"/>
-    <g>
-      <rect x="200" y="80"  width="160" height="160" fill="lime"/>
-      <rect x="160" y="160" width="160" height="160" fill="blue"/>
+    <g style="isolation: isolate">
+      <rect x="120" y="80"  width="160" height="160" fill="red"/>
+      <g style="isolation: isolate">
+        <rect x="200" y="80"  width="160" height="160" fill="lime"/>
+        <rect x="160" y="160" width="160" height="160" fill="blue"/>
+      </g>
     </g>
 
     <!-- Stroke to prevent aliasing from effecting results. -->
-    <g style="mix-blend-mode:normal;fill:none;stroke:black;stroke-width:2px">
+    <g style="fill:none;stroke:black;stroke-width:2px" id="edgecover">
       <rect x="120" y="80"  width="160" height="160"/>
       <rect x="200" y="80"  width="160" height="160"/>
       <rect x="160" y="160" width="160" height="160"/>