[PA] Enable ExternalMetadata feature on canary

Enable ExternalMetadata on canary/dev for 1 day and see how the feature
affects chrome, e.g. stability, performance. Its revert CL will be
created soon after landing this CL. The revert CL will be landed in 1
day.

Bug: crbug.com/435341877
Change-Id: Iff8666b7edb356a4b038090235a8835bb19c0d08
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6824147
Reviewed-by: Yuki Shiino <[email protected]>
Commit-Queue: Takashi Sakamoto <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1498030}
NOKEYCHECK=True
GitOrigin-RevId: 5ecb57d4413475be4daaa1788b9fd074ab0c8bac

Author: tasak

partition_alloc.gni

@@ -340,12 +340,11 @@ declare_args() {
 declare_args() {
   # Because of address space issue, this feature cannot be enabled on
   # 32bit OS and iOS, i.e. has_64_bit_pointers && !is_ios
-  move_metadata_outside_gigacage = false && has_64_bit_pointers && !is_ios
+  move_metadata_outside_gigacage = has_64_bit_pointers && !is_ios
 }
 
 declare_args() {
-  enable_move_metadata_outside_gigacage_trial =
-      false && move_metadata_outside_gigacage
+  enable_move_metadata_outside_gigacage_trial = move_metadata_outside_gigacage
 }
 
 assert(!enable_move_metadata_outside_gigacage_trial ||