Revert "clang plugins: default no-members-in-stack-allocated to on"

This reverts commit 2883cd18.

Reason for revert: broke tests; we also need to update 2 baselines:
https://logs.chromium.org/logs/chromium/buildbucket/cr-buildbucket.appspot.com/8882703558916297872/+/steps/package_clang/0/stdout

Original change's description:
> clang plugins: default no-members-in-stack-allocated to on
> 
> Artifacts can be removed after the clang prebuilt is updated.
> 
> Change-Id: I473c22843445745a39d1fc648a7bbcc336631b8f
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2152329
> Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
> Reviewed-by: Anton Bikineev <bikineev@chromium.org>
> Reviewed-by: Nico Weber <thakis@chromium.org>
> Commit-Queue: Johann Koenig <johannkoenig@google.com>
> Cr-Commit-Position: refs/heads/master@{#759880}

TBR=thakis@chromium.org,kouhei@chromium.org,johannkoenig@google.com,bikineev@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Change-Id: Idd66b71f7b71f7ae64ffc8cb4d24160a86cc80f9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2155363Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#760333}

third_party/blink/renderer/BUILD.gn

@@ -116,7 +116,6 @@ config("config") {
       "blink-gc-plugin",
     ]
 
-    # TODO(bikineev): Remove when clang pre-built has been updated.
     # Disallow members in stack allocated classes.
     cflags += [
       "-Xclang",

tools/clang/blink_gc_plugin/BlinkGCPlugin.cpp

@@ -34,9 +34,7 @@ class BlinkGCPluginAction : public PluginASTAction {
       if (arg == "dump-graph") {
         options_.dump_graph = true;
       } else if (arg == "no-members-in-stack-allocated") {
-        // TODO(bikineev): Flag is on by default. Remove after
-        // third_party/blink/renderer/BUILD.gn has been updated.
-        continue;
+        options_.no_members_in_stack_allocated = true;
       } else if (arg == "enable-weak-members-in-unmanaged-classes") {
         options_.enable_weak_members_in_unmanaged_classes = true;
       } else {

tools/clang/blink_gc_plugin/BlinkGCPluginOptions.h

@@ -12,6 +12,9 @@
 struct BlinkGCPluginOptions {
   bool dump_graph = false;
 
+  // If |true|, disallow Member types in stack allocated classes.
+  bool no_members_in_stack_allocated = false;
+
   // Member<T> fields are only permitted in managed classes,
   // something CheckFieldsVisitor verifies, issuing errors if
   // found in unmanaged classes. WeakMember<T> should be treated

tools/clang/blink_gc_plugin/CheckFieldsVisitor.cpp

@@ -78,14 +78,16 @@ void CheckFieldsVisitor::AtValue(Value* edge) {
     return;
   }
 
-  // Members/WeakMembers are prohibited if the host is stack allocated, but
-  // heap collections with Members are okay.
-  if (stack_allocated_host_ && Parent() &&
-      (Parent()->IsMember() || Parent()->IsWeakMember())) {
-    if (!GrandParent() || !GrandParent()->IsCollection()) {
-      invalid_fields_.push_back(
-          std::make_pair(current_, kMemberInStackAllocated));
-      return;
+  if (options_.no_members_in_stack_allocated) {
+    // Members/WeakMembers are prohibited if the host is stack allocated, but
+    // heap collections with Members are okay.
+    if (stack_allocated_host_ && Parent() &&
+        (Parent()->IsMember() || Parent()->IsWeakMember())) {
+      if (!GrandParent() || !GrandParent()->IsCollection()) {
+        invalid_fields_.push_back(
+            std::make_pair(current_, kMemberInStackAllocated));
+        return;
+      }
     }
   }
 
@@ -115,7 +117,8 @@ void CheckFieldsVisitor::AtValue(Value* edge) {
         current_, InvalidSmartPtr(Parent())));
     return;
   }
-  if (Parent()->IsRawPtr() && !stack_allocated_host_) {
+  if (Parent()->IsRawPtr() &&
+      !(stack_allocated_host_ && options_.no_members_in_stack_allocated)) {
     RawPtr* rawPtr = static_cast<RawPtr*>(Parent());
     Error error = rawPtr->HasReferenceType() ?
         kReferencePtrToGCManaged : kRawPtrToGCManaged;