Revert "use colorspace steps to determine legacy blits"

This reverts commit 0919852526.

Reason for revert: speculative: unblock android roll

Original change's description:
> use colorspace steps to determine legacy blits
> 
> Bug: skia:8793
> Change-Id: I1de4bde25f7dcb12175733a3213c43f92410dc4a
> Reviewed-on: https://skia-review.googlesource.com/c/196647
> Commit-Queue: Mike Reed <reed@google.com>
> Reviewed-by: Mike Klein <mtklein@google.com>
> Reviewed-by: Brian Osman <brianosman@google.com>

TBR=mtklein@google.com,brianosman@google.com,reed@google.com

Change-Id: I79a21df9a3c3a9f3127f57131feff0f3e417e642
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: skia:8793
Reviewed-on: https://skia-review.googlesource.com/c/196774
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>

src/core/SkColorSpacePriv.h

@@ -122,17 +122,4 @@ static inline bool is_almost_linear(const skcms_TransferFunction& coeffs) {
 SkColorSpace* sk_srgb_singleton();
 SkColorSpace* sk_srgb_linear_singleton();
 
-/**
- *  Returns true if the combination of src and dst colorspaces result in basically a no-op,
- *  and thus we will generate correct results if we ignore both colorspaces (as we did in the
- *  legacy world of blits).
- *
- *  Some examples:
- *    dst == null           returns true: this is the classic definition of our legacy blits
- *    dst == src            returns true: going through the new process is effectively a no-op
- *    src == null           treats src as sRGB...
- */
-bool sk_can_use_legacy_blits(SkColorSpace* src, SkColorSpace* dst);
-
-
 #endif  // SkColorSpacePriv_DEFINED

src/core/SkColorSpaceXformSteps.cpp

@@ -174,13 +174,3 @@ void SkColorSpaceXformSteps::apply(SkRasterPipeline* p, bool src_is_normalized)
     }
     if (flags.premul) { p->append(SkRasterPipeline::premul); }
 }
-
-//////////////
-
-bool sk_can_use_legacy_blits(SkColorSpace* src, SkColorSpace* dst) {
-    // When considering legacy blits, we only supported premul, so set those here
-    SkAlphaType srcAT = kPremul_SkAlphaType;
-    SkAlphaType dstAT = kPremul_SkAlphaType;
-
-    return SkColorSpaceXformSteps(src, srcAT, dst, dstAT).flags.mask() == 0;
-}

src/shaders/SkShader.cpp

@@ -8,7 +8,6 @@
 #include "SkArenaAlloc.h"
 #include "SkBitmapProcShader.h"
 #include "SkColorShader.h"
-#include "SkColorSpacePriv.h"
 #include "SkColorSpaceXformer.h"
 #include "SkEmptyShader.h"
 #include "SkMallocPixelRef.h"
@@ -111,7 +110,16 @@ SkShaderBase::Context::Context(const SkShaderBase& shader, const ContextRec& rec
 SkShaderBase::Context::~Context() {}
 
 bool SkShaderBase::ContextRec::isLegacyCompatible(SkColorSpace* shaderColorSpace) const {
-    return sk_can_use_legacy_blits(shaderColorSpace, fDstColorSpace);
+    // Compatible means that the shader's (unmodified) colors will blend correctly with the
+    // device in the legacy mode where the colorspaces (of src and dst) are ignored.
+    if (fDstColorSpace == nullptr) {
+        return true;    // untagged dst is by definition compatible with untagged/ignored src
+    }
+    if (shaderColorSpace == nullptr) {
+        // we treat untagged src as being srgb (e.g. SkColor)
+        return fDstColorSpace->isSRGB();
+    }
+    return SkColorSpace::Equals(shaderColorSpace, fDstColorSpace);
 }
 
 const SkMatrix& SkShader::getLocalMatrix() const {