Revert "Fix performance regresion on Mali, while preserving sk_FragCoord.w"

This reverts commit a34e6ea.

Reason for revert: Broken gold images on Adreno 530 and 540

Original change's description:
> Fix performance regresion on Mali, while preserving sk_FragCoord.w
> 
> Bug: skia:
> Change-Id: I9269288bbb861cd02efa7b5dfe2c9434ec39064e
> Reviewed-on: https://skia-review.googlesource.com/c/174309
> Commit-Queue: Michael Ludwig <michaelludwig@google.com>
> Reviewed-by: Brian Salomon <bsalomon@google.com>

TBR=bsalomon@google.com,michaelludwig@google.com

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: skia:
Change-Id: Id344219c101564a5fd06477895386fad2d8dfa78
Reviewed-on: https://skia-review.googlesource.com/c/175986
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>

Author: lhkbob

src/sksl/SkSLGLSLCodeGenerator.cpp

@@ -756,15 +756,9 @@ void GLSLCodeGenerator::writeFragCoord() {
         this->write("gl_FragCoord");
     } else {
         if (!fSetupFragPositionLocal) {
-            // Accessing the components of gl_FragCoord cause pretty noticeable performance hits on
-            // the Mali400. Reading from the z component of gl_FragCoord or _sktmpCoord, flipping
-            // the y value after assigning to sk_FragCoord, or not using a temp variable are not as
-            // optimal.
             fFunctionHeader += usesPrecisionModifiers() ? "highp " : "";
-            fFunctionHeader += "    vec4 _sktmpCoord = gl_FragCoord;\n";
-            fFunctionHeader += usesPrecisionModifiers() ? "highp " : "";
-            fFunctionHeader += "    vec4 sk_FragCoord = vec4(_sktmpCoord.x, " SKSL_RTHEIGHT_NAME
-                               " - _sktmpCoord.y, 0.0, _sktmpCoord.w);\n";
+            fFunctionHeader += "    vec4 sk_FragCoord = vec4(gl_FragCoord.x, " SKSL_RTHEIGHT_NAME
+                               " - gl_FragCoord.y, gl_FragCoord.z, gl_FragCoord.w);\n";
             fSetupFragPositionLocal = true;
         }
         this->write("sk_FragCoord");

tests/SkSLGLSLTest.cpp

@@ -1147,9 +1147,8 @@ DEF_TEST(SkSLFragCoord, r) {
          "uniform float u_skRTHeight;\n"
          "out vec4 sk_FragColor;\n"
          "void main() {\n"
-         "    vec4 _sktmpCoord = gl_FragCoord;\n"
-         "    vec4 sk_FragCoord = vec4(_sktmpCoord.x, u_skRTHeight - _sktmpCoord.y, "
-                 "0.0, _sktmpCoord.w);\n"
+         "    vec4 sk_FragCoord = vec4(gl_FragCoord.x, u_skRTHeight - gl_FragCoord.y, "
+                 "gl_FragCoord.z, gl_FragCoord.w);\n"
          "    sk_FragColor.xy = sk_FragCoord.xy;\n"
          "}\n",
          &inputs);