base/numerics: Add #define to allow excluding ostream

Add a define to help make base/numerics suitable for use in constrained
environments.

BASE_NUMERICS_DISABLE_OSTREAM_OPERATORS removes a few uses of ostream.

This is motivated by attempting to use base/numerics in the Fuchsia
kernel. ostream is simply unavailable, but what's lost is fairly minor.

This is part of a downstream CL from johngro@:
https://fuchsia-review.googlesource.com/c/fuchsia/+/292396

Change-Id: I1d7a2651e50d8e62c143b048f253b1f4c32971cc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1669874Reviewed-by: Justin Schuh <jschuh@chromium.org>
Commit-Queue: Scott Graham <scottmg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#671366}

base/numerics/clamped_math.h

@@ -192,12 +192,14 @@ constexpr ClampedNumeric<typename UnderlyingType<T>::type> MakeClampedNum(
   return value;
 }
 
+#if !BASE_NUMERICS_DISABLE_OSTREAM_OPERATORS
 // Overload the ostream output operator to make logging work nicely.
 template <typename T>
 std::ostream& operator<<(std::ostream& os, const ClampedNumeric<T>& value) {
   os << static_cast<T>(value);
   return os;
 }
+#endif
 
 // These implement the variadic wrapper for the math operations.
 template <template <typename, typename, typename> class M,

base/numerics/safe_conversions.h

@@ -8,7 +8,6 @@
 #include <stddef.h>
 
 #include <limits>
-#include <ostream>
 #include <type_traits>
 
 #include "base/numerics/safe_conversions_impl.h"
@@ -20,6 +19,10 @@
 #define BASE_HAS_OPTIMIZED_SAFE_CONVERSIONS (0)
 #endif
 
+#if !BASE_NUMERICS_DISABLE_OSTREAM_OPERATORS
+#include <ostream>
+#endif
+
 namespace base {
 namespace internal {
 
@@ -308,12 +311,14 @@ constexpr StrictNumeric<typename UnderlyingType<T>::type> MakeStrictNum(
   return value;
 }
 
+#if !BASE_NUMERICS_DISABLE_OSTREAM_OPERATORS
 // Overload the ostream output operator to make logging work nicely.
 template <typename T>
 std::ostream& operator<<(std::ostream& os, const StrictNumeric<T>& value) {
   os << static_cast<T>(value);
   return os;
 }
+#endif
 
 #define BASE_NUMERIC_COMPARISON_OPERATORS(CLASS, NAME, OP)              \
   template <typename L, typename R,                                     \