Fold alpha to the inner savelayer in savelayer-savelayer-restore
patterns such as this:
SaveLayer (non-opaque)
Save
ClipRect
SaveLayer
Restore
Restore
Restore
Current blink generates these for example for SVG content such as this:
<path style="opacity:0.5 filter:url(#blur_filter)"/>
The outer save layer is due to the opacity and the inner one is due to
blur filter being implemented with picture image filter.
Reduces layers in desk_carsvg.skp testcase from 115 to 78.
BUG=skia:3119
Review URL: https://codereview.chromium.org/835973005
