Refactored SkColorSpace and added in a Lab PCS GM

src/gpu/GrColorSpaceXform.cpp

Index: src/gpu/GrColorSpaceXform.cpp
diff --git a/src/gpu/GrColorSpaceXform.cpp b/src/gpu/GrColorSpaceXform.cpp
index d2270fafdd4a2458b4f8a1721be0a4f41f5adf26..b5404a6fb66c6a54462f7a90a80aab4fb684c18d 100644
--- a/src/gpu/GrColorSpaceXform.cpp
+++ b/src/gpu/GrColorSpaceXform.cpp
@@ -50,7 +50,11 @@ sk_sp<GrColorSpaceXform> GrColorSpaceXform::Make(SkColorSpace* src, SkColorSpace
     }
 
     SkMatrix44 srcToDst(SkMatrix44::kUninitialized_Constructor);
-    srcToDst.setConcat(as_CSB(dst)->fromXYZD50(), as_CSB(src)->toXYZD50());
+    const SkMatrix44* toXYZD50   = as_CSB(src)->toXYZD50();

[msarett, 2016/10/14 21:31:40]

Asserts in test code are good.  Asserts in *unreachable* code are also good. 
Unfortuntately, I realize now that this code is reachable with A2B profiles. 
Clients can make whatever they want with NewICC() and then send them here.  For
that reason, we need to fail gracefully.

const SkMatrix44* toXYZD50   = as_CSB(src)->toXYZD50();
const SkMatrix44* fromXYZD50 = as_CSB(dst)->fromXYZD50();
if (!toXYZD50 || !fromXYZD50) {
    return nullptr;
}

Hopefully we haven't missed any other places where we need to fail nicely?

[raftias, 2016/10/17 15:28:02]

Done. Everywhere else seems to either be test code, tools, or SkColorSpace*
related stuff that is aware of the types. Besides the SampleApp.cpp that checks
the monitor space, I think most are definitely going to be XYZ color spaces.
There was one that loaded an external ICC profile, but it was a XYZ ICC profile.
Besides that I think there were 2 spots in test code where it wasn't known
(CodecTest and TestConfigParsing?), but I think that TestConfigParsing calls
into a place where it just returns SRGB or Linear SRGB, since I hit an error
there when I forgot a line (resulting in a null colorspace) in somewhere I was
setting the colorspace to Linear SRGB.

+    SkASSERT(toXYZD50);
+    const SkMatrix44* fromXYZD50 = as_CSB(dst)->fromXYZD50();
+    SkASSERT(fromXYZD50);
+    srcToDst.setConcat(*fromXYZD50, *toXYZD50);
 
     if (matrix_is_almost_identity(srcToDst)) {
         return nullptr;