Add a fix to base/numerics so that code compiles for asmjs.

Bug: pdfium:1320
Change-Id: I766c1207b42822b58ba1c142d5ae34017880fd9b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1648777Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Andrew Weintraub <asweintraub@google.com>
Cr-Commit-Position: refs/heads/master@{#667254}

base/numerics/safe_math_shared_impl.h

@@ -17,12 +17,15 @@
 
 #include "base/numerics/safe_conversions.h"
 
+#ifdef __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.
-#if !defined(__native_client__) &&                         \
-    ((defined(__clang__) &&                                \
-      ((__clang_major__ > 3) ||                            \
-       (__clang_major__ == 3 && __clang_minor__ >= 4))) || \
-     (defined(__GNUC__) && __GNUC__ >= 5))
+#elif !defined(__native_client__) &&                         \
+      ((defined(__clang__) &&                                \
+        ((__clang_major__ > 3) ||                            \
+         (__clang_major__ == 3 && __clang_minor__ >= 4))) || \
+       (defined(__GNUC__) && __GNUC__ >= 5))
 #include "base/numerics/safe_math_clang_gcc_impl.h"
 #define BASE_HAS_OPTIMIZED_SAFE_MATH (1)
 #else