GN: Enable embedder to decide whether or not the external V8 snapshot is enabled.

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 1c2c52a97ec2294c88c152da6f7e578da8a2204e..9dfd793e530616797e7fdba3849a5d8701f46fbe 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -2,6 +2,10 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+# Because standalone V8 builds are not supported, assume this is part of a
+# Chromium build.
+import("//build/config/features.gni")
+
 # TODO(jochen): These will need to be user-settable to support standalone V8
 # builds.
 v8_compress_startup_data = "off"
@@ -15,7 +19,6 @@ v8_interpreted_regexp = false
 v8_object_print = false
 v8_postmortem_support = false
 v8_use_snapshot = true
-v8_use_external_startup_data = false
 v8_enable_extra_checks = is_debug
 v8_target_arch = cpu_arch
 v8_random_seed = "314159265"
@@ -1412,18 +1415,21 @@ component("v8") {
     "src/v8dll-main.cc",
   ]
 
+  deps = []

[rmcilroy, 2014/11/24 17:21:06]

Is there any reason you need to make these changes?  I would have thought the
if/else if/else structure before would have given you the same result?

[baixo1, 2014/11/24 17:28:11]

I thought the same :). But no :)
When you have the structure:

if (v8_use_external_startup_data)
  do something;
else if (v8_snapshot)
  do something else;
else
  do another thing;

if v8_use_external_startup_data is true, it won't consider the other branches
exist. And then it'll complain that v8_snapshot was defined but never used :)

[rmcilroy, 2014/11/24 18:01:34]

Eww, yuck... Ohh well, perhaps you could just make it 
  if (v8_use_snapshot && v8_use_external_startup_data) {
Since I think v8_use_snapshot has to be true for v8_use_external_startup_data to
be used (and you could assert !v8_use_external_startup_data in the final "else"
branch.

[baixo1, 2014/11/24 21:35:13]

Done.

   if (v8_use_external_startup_data) {
-    deps = [
+    deps += [
       ":v8_base",
       ":v8_external_snapshot",
     ]
-  } else if (v8_use_snapshot) {
-    deps = [
+  }
+  if (v8_use_snapshot && !v8_use_external_startup_data) {
+    deps += [
       ":v8_base",
       ":v8_snapshot",
     ]
-  } else {
-    deps = [
+  }
+  if (!v8_use_snapshot && !v8_use_external_startup_data) {
+    deps += [
       ":v8_base",
       ":v8_nosnapshot",
     ]
@@ -1444,18 +1450,21 @@ component("v8") {
 } else {
 
 group("v8") {
+  deps = []

[rmcilroy, 2014/11/24 17:21:06]

ditto

   if (v8_use_external_startup_data) {
-    deps = [
+    deps += [
       ":v8_base",
       ":v8_external_snapshot",
     ]
-  } else if (v8_use_snapshot) {
-    deps = [
+  }
+  if (v8_use_snapshot && !v8_use_external_startup_data) {
+    deps += [
       ":v8_base",
       ":v8_snapshot",
     ]
-  } else {
-    deps = [
+  }
+  if (!v8_use_snapshot && !v8_use_external_startup_data) {
+    deps += [
       ":v8_base",
       ":v8_nosnapshot",
     ]