Make RLZ build targets exist only when `enable_rlz_support`.

These targets fail to build or link if depended upon without
`enable_rlz_support`. It is a stronger and more relevant condition than
`!is_android`, see: rlz/buildflags/buildflags.gni

Split out from crrev.com/c/2383400

Bug: 898837
Change-Id: Ie675921f61c626eb9a8808df335bc88f309ea31a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2409073
Auto-Submit: Glen Robertson <glenrob@chromium.org>
Commit-Queue: Roger Tawa <rogerta@chromium.org>
Reviewed-by: Roger Tawa <rogerta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806646}

rlz/BUILD.gn

@@ -5,6 +5,7 @@
 # Note that this build file assumes rlz_use_chrome_net which is a condition in
 # the GYP file, but is always true for Chrome builds.
 
+import("//rlz/buildflags/buildflags.gni")
 import("//testing/test.gni")
 
 config("rlz_lib_config") {
@@ -32,7 +33,7 @@ source_set("rlz_utils") {
   deps = [ "//third_party/zlib" ]
 }
 
-if (!is_android) {
+if (enable_rlz_support) {
   source_set("rlz_lib_no_network") {
     sources = [
       "chromeos/lib/rlz_value_store_chromeos.cc",