Make the snapshot a public dependency of v8.

Adds a chain of public dependencies from the v8 target to the snapshot target.
A future version of GN will validate that any files taken as inputs to a step
were generated by public dependencies of that target. Some targets in Chrome
depend on the results of the snapshot.

Review URL: https://codereview.chromium.org/1138953007

Cr-Commit-Position: refs/heads/master@{#28478}

BUILD.gn

@@ -475,9 +475,13 @@ source_set("v8_snapshot") {
     ":js2c",
     ":js2c_experimental",
     ":js2c_extras",
-    ":run_mksnapshot",
     ":v8_base",
   ]
+  public_deps = [
+    # This should be public so downstream targets can declare the snapshot
+    # output file as their inputs.
+    ":run_mksnapshot",
+  ]
 
   sources = [
     "$target_gen_dir/libraries.cc",
@@ -503,9 +507,11 @@ if (v8_use_external_startup_data) {
       ":js2c",
       ":js2c_experimental",
       ":js2c_extras",
-      ":run_mksnapshot",
       ":v8_base",
+    ]
+    public_deps = [
       ":natives_blob",
+      ":run_mksnapshot",
     ]
 
     sources = [
@@ -1574,6 +1580,10 @@ if (component_mode == "shared_library") {
     } else if (v8_use_snapshot) {
       deps = [
         ":v8_base",
+      ]
+      # v8_snapshot should be public so downstream targets can declare the
+      # snapshot file as their input.
+      public_deps = [
         ":v8_snapshot",
       ]
     } else {
@@ -1609,6 +1619,8 @@ if (component_mode == "shared_library") {
     } else if (v8_use_snapshot) {
       deps = [
         ":v8_base",
+      ]
+      public_deps = [
         ":v8_snapshot",
       ]
     } else {