Fix our GN files to build pure objcc targets correctly

This looks like a no-op, because we're just moving flags from cflags_cc
to cflags_objcc, and then removing cflags_cc from the compiler command.
However: GN is very specific about which flags it expects to be used by
the toolchain for each file type. It ONLY emits cflags_cc into ninja
files if there is at least one C++ source file in the target. If we had
a target that only contained .mm source files, cflags_cc would be empty
and the various flags it contains would be missing. Note that ninja
silently leaves undefined variables as empty, making this not obvious.

Bug: skia:13272
Change-Id: I8d729699b1bd34b677ec1b657bc34111fa8db2a4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/534763
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>

gn/skia/BUILD.gn

@@ -52,6 +52,7 @@ config("default") {
   cflags = []
   cflags_c = []
   cflags_cc = []
+  cflags_objcc = []
   defines = []
   ldflags = []
   libs = []
@@ -335,6 +336,8 @@ config("default") {
       cflags_cc += [ "-fno-aligned-allocation" ]
     }
   }
+
+  cflags_objcc += cflags_cc
 }
 
 # See skia:9731.
@@ -346,6 +349,7 @@ config("no_exceptions") {
   # Exceptions are disabled by default on Windows.  (Use /EHsc to enable them.)
   if (!is_win) {
     cflags_cc = [ "-fno-exceptions" ]
+    cflags_objcc = cflags_cc
   }
 }
 
@@ -539,6 +543,8 @@ config("warnings") {
       "-Wdeprecated-writable-str",
     ]
   }
+
+  cflags_objcc += cflags_cc
 }
 config("warnings_for_public_headers") {
   if (is_clang) {
@@ -584,6 +590,7 @@ config("no_rtti") {
       cflags_cc = [ "/GR-" ]
     } else {
       cflags_cc = [ "-fno-rtti" ]
+      cflags_objcc = cflags_cc
     }
   }
 }

gn/toolchain/BUILD.gn

@@ -275,7 +275,7 @@ template("gcc_like_toolchain") {
 
     tool("objcxx") {
       depfile = "{{output}}.d"
-      command = "$cc_wrapper $cxx -MD -MF $depfile {{defines}} {{include_dirs}} {{framework_dirs}} {{cflags}} {{cflags_cc}} {{cflags_objcc}} -c {{source}} -o {{output}}"
+      command = "$cc_wrapper $cxx -MD -MF $depfile {{defines}} {{include_dirs}} {{framework_dirs}} {{cflags}} {{cflags_objcc}} -c {{source}} -o {{output}}"
       depsformat = "gcc"
       outputs =
           [ "{{source_out_dir}}/{{target_output_name}}.{{source_name_part}}.o" ]