Add comment for gcc compiler bug work-around for SK_COMPILE_ASSERT.

If this comment had existed previously, I could have avoided breaking
some bots.

R=bsalomon@google.com

Review URL: https://codereview.chromium.org/196473013

git-svn-id: http://skia.googlecode.com/svn/trunk@13793 2bbb7eff-a529-9590-31e7-b0007b416f81

include/core/SkTypes.h

@@ -129,6 +129,10 @@ struct SkCompileAssert {
 
 // Uses static_cast<bool>(expr) instead of bool(expr) due to
 // https://connect.microsoft.com/VisualStudio/feedback/details/832915
+
+// The extra parentheses in SkCompileAssert<(...)> are a work around for
+// http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57771
+// which was fixed in gcc 4.8.2.
 #define SK_COMPILE_ASSERT(expr, msg) \
     typedef SkCompileAssert<(static_cast<bool>(expr))> \
             msg[static_cast<bool>(expr) ? 1 : -1] SK_UNUSED