Specify empty public for snapshot compile target

By specifying empty list in public header, unnecessary build dependency will be
removed from generated ninja files.

Without this patch, building chrome with -j1000 on goma took 5m18.324s
build trace: https://chromium-build-stats.appspot.com/ninja_log/upload/ninja_log.7WrwKlEIthV27EMferothKEZjCxH-pZFA4BSrqUB5yw=.gz/trace.html

With this patch, building chrome with -j1000 on goma took 4m55.310s
build trace: https://chromium-build-stats.appspot.com/ninja_log/upload/ninja_log.AeodnJ70OlK0PI1FRYndJlvRw1774x4_e3k-j_o0Arc=.gz/trace.html

This patch deserializes dependency between v8 and chrome.

More detail in GN reference.
https://chromium.googlesource.com/chromium/src/+/master/tools/gn/docs/reference.md#public

Bug: chromium:578477
Change-Id: Ia76585e31463d7ea3756b1d5ea30f327801141ef
Reviewed-on: https://chromium-review.googlesource.com/1034173
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52896}

BUILD.gn

@@ -1117,6 +1117,9 @@ if (v8_use_snapshot && !v8_use_external_startup_data) {
       ":run_mksnapshot_default",
     ]
 
+    # Do not publicize any header to remove build dependency.
+    public = []
+
     sources = [
       "$target_gen_dir/experimental-extras-libraries.cc",
       "$target_gen_dir/extras-libraries.cc",
@@ -1167,6 +1170,9 @@ if (v8_use_snapshot && v8_use_external_startup_data) {
       "src/snapshot/snapshot-external.cc",
     ]
 
+    # Do not publicize any header to remove build dependency.
+    public = []
+
     if (v8_enable_embedded_builtins) {
       sources += [ "$target_gen_dir/embedded.cc" ]