[csa] Fix MSVC handling of VA_ARGS in CSA_ASSERT

MSVC's macro VA_ARGS support behaves slightly differently from
gcc/clang, where VA_ARGS is treated as a single token when passed to
other macros, e.g.

    #define FIRST(X, ...) 'X'
    #define FOO(...) FIRST(__VA_ARGS__)
    FOO(a,b,c)

expands to

    gcc/clang: 'a'
    MSVC: 'a,b,c'

The workaround to this is to wrap the call in a no-op macro, which
expands VA_ARGS first, and only then passes it through:

    #define EXPAND(x) x
    #define FOO(...) EXPAND(FIRST(__VA_ARGS__))

This was causing errors on windows builds when CSA_ASSERT was passed
multiple additional expressions.

Change-Id: Ia40bf23baf97af29c7f6f67c8a83918ecca15364
Reviewed-on: https://chromium-review.googlesource.com/586831Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46927}

src/base/macros.h

@@ -9,6 +9,8 @@
 #include "src/base/format-macros.h"
 #include "src/base/logging.h"
 
+// No-op macro which is used to work around MSVC's funky VA_ARGS support.
+#define EXPAND(x) x
 
 // TODO(all) Replace all uses of this macro with C++'s offsetof. To do that, we
 // have to make sure that only standard-layout types and simple field

src/code-stub-assembler.h

@@ -1766,9 +1766,9 @@ class ToDirectStringAssembler : public CodeStubAssembler {
 // We have to jump through some hoops to allow <extra values to print...> to be
 // empty.
 #define CSA_ASSERT(csa, ...)                                                   \
-  (csa)->Assert([&] { return CSA_ASSERT_GET_CONDITION(__VA_ARGS__); },         \
-                CSA_ASSERT_GET_CONDITION_STR(__VA_ARGS__), __FILE__, __LINE__, \
-                CSA_ASSERT_STRINGIFY_EXTRA_VALUES(__VA_ARGS__))
+  (csa)->Assert([&] { return EXPAND(CSA_ASSERT_GET_CONDITION(__VA_ARGS__)); }, \
+                EXPAND(CSA_ASSERT_GET_CONDITION_STR(__VA_ARGS__)), __FILE__,   \
+                __LINE__, CSA_ASSERT_STRINGIFY_EXTRA_VALUES(__VA_ARGS__))
 
 #define CSA_ASSERT_JS_ARGC_OP(csa, Op, op, expected)                      \
   (csa)->Assert(                                                          \