[gn] Set correct defaults for some gn args

This sets default values for v8_enable_gdbjit and
v8_imminent_deprecation_warnings, while allowing to override
them as gn args.

Without this patch the project-specific overrides have no
effect.

BUG=chromium:474921, chromium:626064

Review-Url: https://codereview.chromium.org/2153753002
Cr-Commit-Position: refs/heads/master@{#37797}

BUILD.gn

@@ -26,7 +26,7 @@ declare_args() {
   v8_deprecation_warnings = false
 
   # Enable compiler warnings when using V8_DEPRECATE_SOON apis.
-  v8_imminent_deprecation_warnings = false
+  v8_imminent_deprecation_warnings = ""
 
   # Embeds the given script into the snapshot.
   v8_embed_script = ""
@@ -38,7 +38,7 @@ declare_args() {
   v8_enable_disassembler = false
 
   # Sets -dENABLE_GDB_JIT_INTERFACE.
-  v8_enable_gdbjit = false
+  v8_enable_gdbjit = ""
 
   # Sets -dENABLE_HANDLE_ZAPPING.
   v8_enable_handle_zapping = true
@@ -69,6 +69,23 @@ declare_args() {
   v8_use_mips_abi_hardfloat = true
 }
 
+# Set project-specific defaults for some args if not provided in args.gn. The
+# defaults can be set in the respective build_overrides files.
+if (v8_imminent_deprecation_warnings == "") {
+  if (defined(v8_imminent_deprecation_warnings_default)) {
+    v8_imminent_deprecation_warnings = v8_imminent_deprecation_warnings_default
+  } else {
+    v8_imminent_deprecation_warnings = false
+  }
+}
+if (v8_enable_gdbjit == "") {
+  if (defined(v8_enable_gdbjit_default)) {
+    v8_enable_gdbjit = v8_enable_gdbjit_default
+  } else {
+    v8_enable_gdbjit = false
+  }
+}
+
 v8_random_seed = "314159265"
 v8_toolset_for_shell = "host"
 

build_overrides/v8.gni

@@ -16,10 +16,10 @@ if (((v8_target_cpu == "x86" ||
       v8_target_cpu=="x87") &&
      (is_linux || is_mac)) ||
     (v8_target_cpu == "ppc64" && is_linux)) {
-  v8_enable_gdbjit = true
+  v8_enable_gdbjit_default = true
 }
 
-v8_imminent_deprecation_warnings = true
+v8_imminent_deprecation_warnings_default = true
 
 # Add simple extras solely for the purpose of the cctests.
 v8_extra_library_files = [ "//test/cctest/test-extra.js" ]