Fix radical assumption that 0.5 in double is exactly convertible to float.

BUG=skia:2181

Review URL: https://codereview.chromium.org/195423002

git-svn-id: http://skia.googlecode.com/svn/trunk@13742 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/src/gpu/effects/GrRRectEffect.cpp b/src/gpu/effects/GrRRectEffect.cpp
index 619cd99..1bbf2f7 100644
--- a/src/gpu/effects/GrRRectEffect.cpp
+++ b/src/gpu/effects/GrRRectEffect.cpp
@@ -314,23 +314,23 @@
                 break;
             case RRectEffect::kTopRight_CornerFlag:
                 radius = rrect.radii(SkRRect::kUpperRight_Corner).fX;
-                rect.fLeft -= 0.5;
+                rect.fLeft -= 0.5f;
                 rect.fTop += radius;
                 rect.fRight -= radius;
                 rect.fBottom += 0.5f;
                 break;
             case RRectEffect::kBottomRight_CornerFlag:
                 radius = rrect.radii(SkRRect::kLowerRight_Corner).fX;
-                rect.fLeft -= 0.5;
-                rect.fTop -= 0.5;
+                rect.fLeft -= 0.5f;
+                rect.fTop -= 0.5f;
                 rect.fRight -= radius;
                 rect.fBottom -= radius;
                 break;
             case RRectEffect::kBottomLeft_CornerFlag:
                 radius = rrect.radii(SkRRect::kLowerLeft_Corner).fX;
                 rect.fLeft += radius;
-                rect.fTop -= 0.5;
-                rect.fRight += 0.5;
+                rect.fTop -= 0.5f;
+                rect.fRight += 0.5f;
                 rect.fBottom -= radius;
                 break;
             case RRectEffect::kLeft_CornerFlags: