Revert of Remove relative path to GrColor.h in SkShader.h (https://codereview.chromium.org/422023005/)

Reason for revert:
As Ben suspected, this breaks the chrome build.

Original issue's description:
> Remove relative path to GrColor.h in SkShader.h
>
> Committed: https://skia.googlesource.com/skia/+/939f430f660d26e46116a38d2a8436afbd55dfb9

R=bungeman@google.com, george@mozilla.com
TBR=bungeman@google.com, george@mozilla.com
NOTREECHECKS=true
NOTRY=true

Author: bsalomon@google.com

Review URL: https://codereview.chromium.org/423943005

gyp/core.gyp

@@ -15,7 +15,6 @@
       'include_dirs': [
         '../include/config',
         '../include/core',
-        '../include/gpu',
         '../include/pathops',
         '../include/pipe',
         '../include/ports',
@@ -92,7 +91,8 @@
         }],
         ['skia_gpu == 1', {
           'include_dirs': [
-            '../src/gpu',
+              '../include/gpu',
+              '../src/gpu',
           ],
         }],
       ],
@@ -100,7 +100,6 @@
         'include_dirs': [
           '../include/config',
           '../include/core',
-          '../include/gpu',
           '../include/pathops',
           '../include/pipe',
         ],

include/core/SkShader.h

@@ -14,7 +14,7 @@
 #include "SkMask.h"
 #include "SkMatrix.h"
 #include "SkPaint.h"
-#include "GrColor.h"
+#include "../gpu/GrColor.h"
 
 class SkPath;
 class SkPicture;