Fix iOS build

BUG=skia:

Change-Id: I914cd75c84bbe57401ab65352d4e8b3dc413dce1
Reviewed-on: https://skia-review.googlesource.com/6418
Reviewed-by: Ravi Mistry <rmistry@google.com>
Commit-Queue: Matt Sarett <msarett@google.com>
diff --git a/gm/readpixels.cpp b/gm/readpixels.cpp
index 26d8e73..024cb43 100644
--- a/gm/readpixels.cpp
+++ b/gm/readpixels.cpp
@@ -63,7 +63,7 @@
 }
 
 static sk_sp<SkColorSpace> make_srgb_transfer_fn(const SkColorSpacePrimaries& primaries) {
-    SkMatrix44 toXYZD50;
+    SkMatrix44 toXYZD50(SkMatrix44::kUninitialized_Constructor);
     SkAssertResult(primaries.toXYZD50(&toXYZD50));
     return SkColorSpace::MakeRGB(SkColorSpace::kSRGB_RenderTargetGamma, toXYZD50);
 }