[cleanup] Fix use of BooleanValue

Replace use of deprecated Maybe<bool> BooleanValue(Context) with bool
BooleanValue(Isolate*). The Maybe version is unnecessary as BooleanValue
can never throw.

Bug: v8:7279, v8:8238
Change-Id: Iaf827cedbf158c59727900ff551962e2af4ccbac
Reviewed-on: https://chromium-review.googlesource.com/c/1344063
Commit-Queue: Dan Elphick <delphick@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609765}

gin/converter.cc

@@ -44,7 +44,9 @@ Local<Value> Converter<bool>::ToV8(Isolate* isolate, bool val) {
 }
 
 bool Converter<bool>::FromV8(Isolate* isolate, Local<Value> val, bool* out) {
-  return FromMaybe(val->BooleanValue(isolate->GetCurrentContext()), out);
+  *out = val->BooleanValue(isolate);
+  // BooleanValue cannot throw.
+  return true;
 }
 
 Local<Value> Converter<int32_t>::ToV8(Isolate* isolate, int32_t val) {