PSM: Support PSM libs on ChromeOS platform only

In that way, PSM libs are being supported on ChromeOS only. However,
due to the current device_management_backend structure, PSM protos
can be used by multiple platforms too.

Bug: chromium:1124157
Change-Id: I0e9e6f2f2c55a5088ef1484262ada3f984ff5b88
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2389202Reviewed-by: Yves Arrouye <drcrash@chromium.org>
Reviewed-by: Amr Aboelkher <amraboelkher@google.com>
Reviewed-by: Amr Aboelkher <amraboelkher@chromium.org>
Commit-Queue: Amr Aboelkher <amraboelkher@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803784}

third_party/private_membership/BUILD.gn

@@ -34,7 +34,8 @@ proto_library("private_membership_proto") {
   link_deps = [ "//third_party/shell-encryption:serialization_proto" ]
 }
 
-source_set("private_membership") {
+if (is_chromeos) {
+  source_set("private_membership") {
     public_configs = [ ":private_membership_config" ]
     configs -= [ "//build/config/compiler:chromium_code" ]
     configs += [ "//build/config/compiler:no_chromium_code" ]
@@ -73,6 +74,7 @@ source_set("private_membership") {
       "//third_party/private-join-and-compute/src:ec_commutative_cipher",
       "//third_party/shell-encryption:shell_encryption",
     ]
+  }
 }
 
 if (is_chromeos) {

third_party/shell-encryption/BUILD.gn

@@ -46,7 +46,8 @@ proto_library("coefficient_polynomial_proto") {
 }
 
 # SHELL lib.
-source_set("shell_encryption") {
+if (is_chromeos) {
+  source_set("shell_encryption") {
     public_configs = [ ":shell_encryption_config1" ]
     configs -= [ "//build/config/compiler:chromium_code" ]
     configs += [ "//build/config/compiler:no_chromium_code" ]
@@ -92,6 +93,7 @@ source_set("shell_encryption") {
       "//third_party/boringssl:boringssl",
       "//third_party/protobuf:protobuf_lite",
     ]
+  }
 }
 
 if (is_chromeos) {