Auto merge of #7277 - pcwalton:position-relative-stacking-context, r=glennw

layout: Make `position: relative` with non-auto `z-index` create a stacking context.

Improves imgur.com and Fast Company articles.

This change made `min-height-106.htm.ini` fail because the thing it was testing
for never worked: we were relying on the incorrect stacking order of `position:
relative` to get the green square to show up.

r? @glennw

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tests/wpt/metadata-css/css21_dev/html4/min-height-106.htm.ini

@@ -1,3 +1,3 @@
-[floats-041.htm]
+[min-height-106.htm]
   type: reftest
   expected: FAIL

tests/wpt/mozilla/tests/mozilla/canvas/drawimage_html_image_5_ref.html

@@ -20,7 +20,6 @@
     left: 50px;
     width: 64px;
     height: 32px;
-    overflow: hidden;
   }
 
   #destination .img img{