Add DAWN_UNUSED_FUNC to ignore unused functions in generated code

BUG=dawn:13

Change-Id: I7a224503d0a33ef148e63b8327a6a53df1b3868a
Reviewed-on: https://dawn-review.googlesource.com/1520
Commit-Queue: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Stephen White <senorblanco@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>

generator/templates/dawn_wire/WireCmd.cpp

@@ -135,6 +135,9 @@
 
         return result;
     }
+    // GetExtraRequiredSize isn't used for structures that are value members of other structures
+    // because we assume they cannot contain pointers themselves.
+    DAWN_UNUSED_FUNC({{name}}GetExtraRequiredSize);
 
     //* Serializes `record` into `transfer`, using `buffer` to get more space for pointed-to data
     //* and `provider` to serialize objects.

src/common/Compiler.h

@@ -23,6 +23,7 @@
 //  - DAWN_(UN)?LIKELY(EXPR): Where available, hints the compiler that the expression will be true
 //      (resp. false) to help it generate code that leads to better branch prediction.
 //  - DAWN_UNUSED(EXPR): Prevents unused variable/expression warnings on EXPR.
+//  - DAWN_UNUSED_FUNC(FUNC): Prevents unused function warnings on FUNC.
 
 // Clang and GCC, check for __clang__ too to catch clang-cl masquarading as MSVC
 #if defined(__GNUC__) || defined(__clang__)
@@ -75,6 +76,8 @@ extern void __cdecl __debugbreak(void);
 
 // It seems that (void) EXPR works on all compilers to silence the unused variable warning.
 #define DAWN_UNUSED(EXPR) (void)EXPR
+// Likewise using static asserting on sizeof(&FUNC) seems to make it tagged as used
+#define DAWN_UNUSED_FUNC(FUNC) static_assert(sizeof(&FUNC) == sizeof(void (*)()), "");
 
 // Add noop replacements for macros for features that aren't supported by the compiler.
 #if !defined(DAWN_LIKELY)