[CSS Blending] Implement the isolation property for SVG.

This patch adds the isolation functionality to SVG, performs
some cleanup on the current blend modes SVG implementation
and adds tests to validate the feature.

BUG=229615

Review URL: https://codereview.chromium.org/108163004

git-svn-id: svn://svn.chromium.org/blink/trunk@164028 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Simplify DocumentOrderedMap

This CL simplifies DocumentOrderedMap's internal state. Rather than keeping a
map separately from the duplicate count, we now keep the duplicate count in the
same map.

This CL is a fixed version of r155823, which was reverted because it was causing
an assertion in clusterfuzz.

R=abarth, adamk
BUG=265009, 271131

Review URL: https://codereview.chromium.org/117313002

git-svn-id: svn://svn.chromium.org/blink/trunk@164029 bbb929c8-8fbe-4397-9dbb-9b2b20218538