raw_ptr: Fix component build

`raw_ptr` cannot be promoted into a component without hitting issues
with undefined CRT symbols. Instead,

1. We assert that the `//base` component provides access to the
   `raw_ptr` source set in Chromium and
2. We provide the appropriate define to make the component build work
   with `HookableRawPtrImpl`.

Fixed: 1413475
Change-Id: Iadd0c13ab3bd768006f972121f27d90ab429bca8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4235042
Auto-Submit: Kalvin Lee <kdlee@chromium.org>
Commit-Queue: Kalvin Lee <kdlee@chromium.org>
Reviewed-by: Keishi Hattori <keishi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1104358}

base/BUILD.gn

@@ -1078,6 +1078,11 @@ component("base") {
     "//third_party/modp_b64",
   ]
 
+  # `raw_ptr` cannot be made a component due to CRT symbol issues.
+  # Its gateway to being a component is through `//base`, so we have
+  # to provide the appropriate `#define` here.
+  defines += [ "IS_RAW_PTR_IMPL" ]
+
   if (is_apple) {
     deps += [ "//base/time/buildflags:buildflags" ]
   }

base/allocator/partition_allocator/BUILD.gn

@@ -398,6 +398,12 @@ source_set("raw_ptr") {
 
   # See also: `partition_alloc_base/component_export.h`
   defines = [ "IS_RAW_PTR_IMPL" ]
+
+  # When built inside Chromium, although this cannot directly be made a
+  # component, we expect `//base` to provide the only GN-level access.
+  if (build_with_chromium) {
+    visibility = [ "//base" ]
+  }
 }
 
 buildflag_header("partition_alloc_buildflags") {