Remove a workaround from stack_protector_test that's not needed in master.

The compiler in master doesn't leave a gap between the buffer and the stack
guard canary.

Bug: http://b/27815668
Change-Id: Iae6681683881055c2719d661a2179f5305c23135

tests/stack_protector_test_helper.cpp

diff --git a/tests/stack_protector_test_helper.cpp b/tests/stack_protector_test_helper.cpp
index 53a5e05..3f15a12 100644
--- a/tests/stack_protector_test_helper.cpp
+++ b/tests/stack_protector_test_helper.cpp
@@ -22,10 +22,5 @@
   // Without volatile, the generic x86/x86-64 targets don't write to the stack.
   volatile char* p = buf;
   int size = static_cast<int>(sizeof(buf) + 1);
-#if __x86_64__
-  // The generic x86-64 target leaves an 8-byte gap between `buf` and the stack guard.
-  // We only need to corrupt one byte though.
-  size += 8;
-#endif
   while ((p - buf) < size) *p++ = '\0';
 }