Use last good version before build error

Using bisect, I was able to find that the build error was introduced in
5bbca548e9,
so let's use the version before that one while we wait for upstream to
merge the patch (https://bugs.chromium.org/p/v8/issues/detail?id=10708)

CHANGELOG.md

@@ -1,3 +1,7 @@
+### v8.4.255.0 - 2020-07-15
+
+* Update upstream v8 version to 8.4.255.0
+
 ### v7.3.495.0 - 2020-04-14
 
 * Update upstream v8 version to 7.3.495.0

ext/libv8/builder.rb

@@ -65,7 +65,7 @@ module Libv8
     # then this will be 4.5.95
     #
     def source_version
-      Libv8::V8_MINOR_VERSION
+      Libv8::VERSION.gsub(/\.[^.]+$/, '')
     end
 
     ##
@@ -84,7 +84,7 @@ module Libv8
 
         Dir.chdir('v8') do
           system 'git fetch origin'
-          unless system "git checkout branch-heads/#{source_version}"
+          unless system "git checkout #{source_version}"
             fail "unable to checkout source for v8 #{source_version}"
           end
           system "gclient sync" or fail "could not sync v8 build dependencies"

lib/libv8/version.rb

@@ -1,4 +1,3 @@
 module Libv8
-  V8_MINOR_VERSION = "8.4"
-  VERSION = "8.4.371.0"
+  VERSION = "8.4.255.0"
 end

vendor/depot_tools

@@ -1 +1 @@
-Subproject commit 3229d944bfb4bc5983785f49cbc58f78872d8cb8
+Subproject commit 9d9199a509bf792d7d24ac91d92ed0d111f8d9fb