[PartitionAlloc] Guard `no_default_deps` with `build_with_chromium`

When PartitionAlloc is embedded in standalone GN builds (like Skia) as
a component build on Windows, the `no_default_deps = true` assignment
fails the GN evaluation with "Assignment had no effect".

This occurs because `no_default_deps` is not a built-in GN variable,
but rather relies on Chromium's wrapper templates to intercept and
use it. Standalone builds that map `component` to a standard
`shared_library` do not consume it.

This CL wraps the assignment in `if (build_with_chromium)` to allow
standalone GN builds to succeed.

Bug: 351867706
Change-Id: Id89392613b78097813b6b462b11795fd3425a67f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7827478
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Reviewed-by: Stephen Nusko <nuskos@chromium.org>
Reviewed-by: Takashi Sakamoto <tasak@google.com>
Auto-Submit: Arthur Sonzogni <arthursonzogni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1629136}
NOKEYCHECK=True
GitOrigin-RevId: a8b5ac23ee4fc343b324fb9d4d54d3e04bbe3897

src/partition_alloc/BUILD.gn

diff --git a/src/partition_alloc/BUILD.gn b/src/partition_alloc/BUILD.gn
index e6c0368..63b0b55 100644
--- a/src/partition_alloc/BUILD.gn
+++ b/src/partition_alloc/BUILD.gn
@@ -947,7 +947,9 @@
 
         # allocator_shim cannot depend on libc++ objects because they use malloc()
         # internally.
-        no_default_deps = true
+        if (build_with_chromium) {
+          no_default_deps = true
+        }
       }
     }