Update from https://crrev.com/327068

sandbox/linux/system_headers/i386_linux_ucontext.h

Index: sandbox/linux/system_headers/i386_linux_ucontext.h
diff --git a/sandbox/linux/system_headers/android_i386_ucontext.h b/sandbox/linux/system_headers/i386_linux_ucontext.h
similarity index 82%
rename from sandbox/linux/system_headers/android_i386_ucontext.h
rename to sandbox/linux/system_headers/i386_linux_ucontext.h
index 868016b5570647413b3b864c0d2b8eece35ef039..61d9f7a9b8abeb1de32d96253c1b3e8d666ab4e8 100644
--- a/sandbox/linux/system_headers/android_i386_ucontext.h
+++ b/sandbox/linux/system_headers/i386_linux_ucontext.h
@@ -11,7 +11,16 @@
 // except we do use sigset_t for uc_sigmask instead of a custom type.
 
 #if !defined(__BIONIC_HAVE_UCONTEXT_T)
+#if !defined(__native_client_nonsfi__)
 #include <asm/sigcontext.h>
+#else
+// In PNaCl toolchain, sigcontext is not defined. So here declare it.
+typedef struct sigaltstack {
+  void* ss_sp;
+  int ss_flags;
+  size_t ss_size;
+} stack_t;
+#endif
 
 /* 80-bit floating-point register */
 struct _libc_fpreg {
@@ -68,7 +77,12 @@ typedef struct ucontext {
   struct ucontext* uc_link;
   stack_t uc_stack;
   mcontext_t uc_mcontext;
-  sigset_t uc_sigmask;
+  // Android and PNaCl toolchain's sigset_t has only 32 bits, though Linux
+  // ABI requires 64 bits.
+  union {
+    sigset_t uc_sigmask;
+    uint32_t kernel_sigmask[2];
+  };
   struct _libc_fpstate __fpregs_mem;
 } ucontext_t;