Revert "[PA] Enable ExternalMetadata feature on canary"

This reverts commit 5ecb57d4413475be4daaa1788b9fd074ab0c8bac.

Reason for revert: disable the feature and analyze the feature's stability.

Bug: crbug.com/435341877
Original change's description:
> [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}

Bug: crbug.com/435341877
Change-Id: I3297dd63d8b07e8f73b9a495c9a2010cfc87ad96
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6827041
Commit-Queue: Takashi Sakamoto <[email protected]>
Bot-Commit: Rubber Stamper <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1498067}
NOKEYCHECK=True
GitOrigin-RevId: 8f6f1fcbe30f6cc8b77840404788d180dc674f5a

Author: tasak

partition_alloc.gni

@@ -340,11 +340,12 @@ 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 = has_64_bit_pointers && !is_ios
+  move_metadata_outside_gigacage = false && has_64_bit_pointers && !is_ios
 }
 
 declare_args() {
-  enable_move_metadata_outside_gigacage_trial = move_metadata_outside_gigacage
+  enable_move_metadata_outside_gigacage_trial =
+      false && move_metadata_outside_gigacage
 }
 
 assert(!enable_move_metadata_outside_gigacage_trial ||