Support WebAssembly in build/build_config.h.

Bug: 804907
Change-Id: I0d67bd2b315d140c3418157bd42b380ce151577d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1716916
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#690619}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 73e66f88573c227e248454cdf1a7770d6dc9aa35

build_config.h

@@ -63,7 +63,7 @@
 #define OS_QNX 1
 #elif defined(_AIX)
 #define OS_AIX 1
-#elif defined(__asmjs__)
+#elif defined(__asmjs__) || defined(__wasm__)
 #define OS_ASMJS
 #else
 #error Please add support for your platform in build/build_config.h
@@ -139,7 +139,7 @@
 #define ARCH_CPU_ARM64 1
 #define ARCH_CPU_64_BITS 1
 #define ARCH_CPU_LITTLE_ENDIAN 1
-#elif defined(__pnacl__) || defined(__asmjs__)
+#elif defined(__pnacl__) || defined(__asmjs__) || defined(__wasm__)
 #define ARCH_CPU_32_BITS 1
 #define ARCH_CPU_LITTLE_ENDIAN 1
 #elif defined(__MIPSEL__)