[PA] Enable PartitionAllocExternalMetadata on canary

Enable the feature to see how the feature affects chrome statiblity
and performance. This CL will be reverted in 1 day (not launch).

Bug: crbug.com/435341877
Change-Id: I3ed9a264a2665bdc3e85cd5451245d532ee0e7e9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6839319
Reviewed-by: Yuki Shiino <[email protected]>
Commit-Queue: Takashi Sakamoto <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1499967}
NOKEYCHECK=True
GitOrigin-RevId: dcc8d784780e322ecf03d75202715d8c407f0832

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 ||