Update SRGBReadWritePixelsTest again, to only test opaque colors

With our new world (never doing linear blending), the read-back
conversion from premul sRGB to premul linear no longer produces the same
answer. By always using opaque pixels, the existing check functions in
this test can continue to verify that the sRGB <-> linear conversion is
happening correctly during read/write, independent of any color space
transformation (which is now altering the outcome).

Bug: skia:
Change-Id: Id8e28b3bc4b9674043bd870a39d229207ccdc6d7
Reviewed-on: https://skia-review.googlesource.com/140987
Auto-Submit: Brian Osman <brianosman@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>

tests/SRGBReadWritePixelsTest.cpp

@@ -188,7 +188,7 @@ static std::unique_ptr<uint32_t[]> make_data() {
     std::unique_ptr<uint32_t[]> data(new uint32_t[kW * kH]);
     for (int j = 0; j < kH; ++j) {
         for (int i = 0; i < kW; ++i) {
-            data[j * kW + i] = (SkTMax(j, i) << 24) | (i << 16) | (i << 8) | i;
+            data[j * kW + i] = (0xFF << 24) | (i << 16) | (i << 8) | i;
         }
     }
     return data;