Optimize build dependency for headless

snapshot is not necessary to be there when compiling.
This change allows src for headless to be compiled before generating snapshot.

This dependency optimization improves build parallelism and build time of chrome on Z840 linux with goma -j1000 reduced from 6m14.035s to 4m45.244s when goma's backend cache cannot be used.

Change-Id: I90e302f4b9da8ec252020755636483006f332a80
Reviewed-on: https://chromium-review.googlesource.com/1013958Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551202}

headless/BUILD.gn

@@ -444,6 +444,9 @@ component("headless") {
     "//net",
   ]
 
+  data_deps = []
+  data = []
+
   if (enable_basic_printing) {
     public_deps += [ "//skia" ]
   }
@@ -520,10 +523,10 @@ component("headless") {
       sources += [ "$root_out_dir/natives_blob.bin" ]
       public_deps += [ "//v8" ]
       if (use_v8_context_snapshot) {
-        sources += [ "$root_out_dir/v8_context_snapshot.bin" ]
-        public_deps += [ "//tools/v8_context_snapshot" ]
+        data += [ "$root_out_dir/v8_context_snapshot.bin" ]
+        data_deps += [ "//tools/v8_context_snapshot" ]
       } else {
-        sources += [ "$root_out_dir/snapshot_blob.bin" ]
+        data += [ "$root_out_dir/snapshot_blob.bin" ]
       }
     }
   }