Consistently define OS_ASMJS and use it in place of __asmjs__.

In build/build_config.h, the #define for OS_ASMJS is inconsistent with
the surround OS_* defines. Make it consistent. Then use it in
base/numerics/safe_math_shared_impl.h instead of using __asmjs__
directly.

Change-Id: Id4605afcbdcff14fa64183eed6be46422c2e4066
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2269764Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#783676}

base/numerics/safe_math_shared_impl.h

@@ -16,8 +16,9 @@
 #include <type_traits>
 
 #include "base/numerics/safe_conversions.h"
+#include "build/build_config.h"
 
-#ifdef __asmjs__
+#if defined(OS_ASMJS)
 // Optimized safe math instructions are incompatible with asmjs.
 #define BASE_HAS_OPTIMIZED_SAFE_MATH (0)
 // Where available use builtin math overflow support on Clang and GCC.

build/build_config.h

@@ -64,7 +64,7 @@
 #elif defined(_AIX)
 #define OS_AIX 1
 #elif defined(__asmjs__) || defined(__wasm__)
-#define OS_ASMJS
+#define OS_ASMJS 1
 #else
 #error Please add support for your platform in build/build_config.h
 #endif