Fix some win64 warnings.

Review URL: https://codereview.chromium.org/795223002
diff --git a/src/gpu/GrDistanceFieldTextContext.cpp b/src/gpu/GrDistanceFieldTextContext.cpp
index d35ba23..641d0c1 100755
--- a/src/gpu/GrDistanceFieldTextContext.cpp
+++ b/src/gpu/GrDistanceFieldTextContext.cpp
@@ -306,7 +306,7 @@
                                                      glyph.getSubYFixed()),
                                        x, y, fontScaler)) {
                     // couldn't append, send to fallback
-                    fallbackTxt.push_back_n(text-lastText, lastText);
+                    fallbackTxt.push_back_n(SkToU32(text-lastText), lastText);
                     fallbackPos.push_back(pos[0]);
                     if (2 == scalarsPerPosition) {
                         fallbackPos.push_back(pos[1]);
@@ -335,7 +335,7 @@
                                                      glyph.getSubYFixed()),
                                        x - advanceX, y - advanceY, fontScaler)) {
                     // couldn't append, send to fallback
-                    fallbackTxt.push_back_n(text-lastText, lastText);
+                    fallbackTxt.push_back_n(SkToU32(text-lastText), lastText);
                     fallbackPos.push_back(pos[0]);
                     if (2 == scalarsPerPosition) {
                         fallbackPos.push_back(pos[1]);