Only add -isystem for libc++ for C++ files, not C files.

Seems cleaner.  libc++ recently added a few headers that shadow
C headers, and shouldn't pick those up when building C files.

No intended behavior change.

BUG=none

Review URL: https://codereview.chromium.org/1647493002

Cr-Commit-Position: refs/heads/master@{#371949}

build/common.gypi

@@ -4829,6 +4829,8 @@
               '-finline-limit=64',
               '<@(release_extra_cflags)',
               '--sysroot=<(android_ndk_sysroot)',
+            ],
+            'cflags_cc': [
               # NOTE: The libc++ header include paths below are specified in
               # cflags rather than include_dirs because they need to come
               # after include_dirs.

build/config/android/BUILD.gn

@@ -123,13 +123,13 @@ config("compiler") {
 # that is Android-only. Please see that target for advice on what should go in
 # :runtime_library vs. :compiler.
 config("runtime_library") {
-  # NOTE: The libc++ header include paths below are specified in cflags
+  # NOTE: The libc++ header include paths below are specified in cflags_cc
   # rather than include_dirs because they need to come after include_dirs.
   # Think of them like system headers, but don't use '-isystem' because the
   # arm-linux-androideabi-4.4.3 toolchain (circa Gingerbread) will exhibit
   # strange errors. The include ordering here is important; change with
   # caution.
-  cflags = [
+  cflags_cc = [
     "-isystem" +
         rebase_path("$android_libcpp_root/libcxx/include", root_build_dir),
     "-isystem" + rebase_path(