Silence ASAN int32 overflow warning

src/effects/gradients/SkLinearGradient.cpp

Index: src/effects/gradients/SkLinearGradient.cpp
diff --git a/src/effects/gradients/SkLinearGradient.cpp b/src/effects/gradients/SkLinearGradient.cpp
index 209b833973291f3b4199299320510935c6f31ecd..7752aac51d5431d281c11aac1ae94ac48d287928 100644
--- a/src/effects/gradients/SkLinearGradient.cpp
+++ b/src/effects/gradients/SkLinearGradient.cpp
@@ -612,7 +612,8 @@ void SkLinearGradient::LinearGradientContext::shade4_dx_clamp(SkPMColor dstC[],
         if (fx < 0) {
             // count is guaranteed to be positive, but the first arg may overflow int32 after
             // increment => casting to uint32 ensures correct clamping.
-            int n = SkTMin<uint32_t>(SkFloatToIntFloor(-fx * invDx) + 1, count);
+            int n = SkTMin<uint32_t>(static_cast<uint32_t>(SkFloatToIntFloor(-fx * invDx)) + 1,

[mtklein, 2016/05/27 05:17:03]

See if I'm following this correctly?

  1) We're a safe int32_t after SkFloatToIntFloor(...), with a max of
0x7fffffff.

  2) Then we cast it to uint32_t and add 1.  This is a safe uint32_t but a
possibly wrapped int32_t (0x8000000).

  3) Then we min it with count as uint32_t.  Is that what makes this safe?  We
know count is in [0, 0x7ffffff] ?

  4) That uint32_t in [0, 0x7fffffff] is finally cast back to a safe int32_t.

+                                     count);
             SkASSERT(n > 0);
             fill<apply_alpha>(dstC, n, rec[0].fColor);
             count -= n;
@@ -627,7 +628,8 @@ void SkLinearGradient::LinearGradientContext::shade4_dx_clamp(SkPMColor dstC[],
         if (fx > 1) {
             // count is guaranteed to be positive, but the first arg may overflow int32 after
             // increment => casting to uint32 ensures correct clamping.
-            int n = SkTMin<uint32_t>(SkFloatToIntFloor((1 - fx) * invDx) + 1, count);
+            int n = SkTMin<uint32_t>(static_cast<uint32_t>(SkFloatToIntFloor((1 - fx) * invDx)) + 1,
+                                     count);
             SkASSERT(n > 0);
             fill<apply_alpha>(dstC, n, rec[fRecs.count() - 1].fColor);
             count -= n;