color: Do not enforce T(0)=0 and T(1)=1 constraints in approxmation

ui/gfx/color_space_unittest.cc

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <cmath>
 
 #include "base/logging.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "ui/gfx/color_space.h"
 #include "ui/gfx/skia_color_space_util.h"
@@ skipping 91 matching lines @@
     t.push_back(SkTransferFnEval(tr_fn, v));
   }
 
   SkColorSpaceTransferFn fn_approx;
   bool converged =
       SkApproximateTransferFn(x.data(), t.data(), x.size(), &fn_approx);
   EXPECT_TRUE(converged);
 
   for (size_t i = 0; i < x.size(); ++i) {
     float fn_approx_of_x = SkTransferFnEval(fn_approx, x[i]);
-    EXPECT_NEAR(t[i], fn_approx_of_x, 2.f / 256.f);
-    if (std::abs(t[i] - fn_approx_of_x) > 2.f / 256.f)
+    EXPECT_NEAR(t[i], fn_approx_of_x, 3.f / 256.f);
+    if (std::abs(t[i] - fn_approx_of_x) > 3.f / 256.f)
       break;
   }
 }
 
 ColorSpace::TransferID all_transfers[] = {
     ColorSpace::TransferID::GAMMA22,   ColorSpace::TransferID::GAMMA24,
     ColorSpace::TransferID::GAMMA28,   ColorSpace::TransferID::BT709,
     ColorSpace::TransferID::SMPTE240M, ColorSpace::TransferID::IEC61966_2_1,
     ColorSpace::TransferID::LINEAR};
 
@@ skipping 43 matching lines @@
       float fn_approx_of_x = SkTransferFnEval(fn_approx, x[i]);
       EXPECT_NEAR(t[i], fn_approx_of_x, expected_error);
       if (std::abs(t[i] - fn_approx_of_x) > expected_error)
         break;
     }
   }
 }
 
 }  // namespace
 }  // namespace gfx