Reland "Enable Snap package for official builds"

This is a reland of d1bd3ffb

Original change's description:
> Enable Snap package for official builds
>
> BUG=1055026,1090915
> R=dpranke
> CC=goanuj
>
> Change-Id: I7009f67268fa5d35e9d41fca593248c36f6ef631
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2232837
> Auto-Submit: Thomas Anderson <thomasanderson@chromium.org>
> Reviewed-by: Dirk Pranke <dpranke@chromium.org>
> Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#776165}

TBR=dpranke

Bug: 1055026, 1090915
Change-Id: I80fd77b927b9a832b779bdb09b843016a9c3a93f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2236069Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Auto-Submit: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#776202}

chrome/installer/linux/BUILD.gn

@@ -24,7 +24,7 @@ declare_args() {
   enable_distro_version_check = true
 
   # Enable building snap packages
-  enable_snap_package = false
+  enable_snap_package = is_official_build
 }
 
 assert(is_linux)