Require `enable_media_drm_storage` for rules in components/cdm/browser/.

These targets fail to build if depended upon without
`enable_media_drm_storage`, so prevent that from being possible.
Split out from crrev.com/c/2383400

Bug: 898837
Change-Id: I7e0b94d81d2a722d93e41188812d532533a0255c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2407420
Auto-Submit: Glen Robertson <glenrob@chromium.org>
Commit-Queue: John Rummell <jrummell@chromium.org>
Reviewed-by: John Rummell <jrummell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806710}

components/cdm/browser/BUILD.gn

@@ -2,7 +2,10 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-source_set("browser") {
+import("//media/media_options.gni")
+
+if (enable_media_drm_storage) {
+  source_set("browser") {
     sources = [
       "media_drm_storage_impl.cc",
       "media_drm_storage_impl.h",
@@ -32,9 +35,9 @@ source_set("browser") {
       ]
       deps += [ "//third_party/widevine/cdm:headers" ]
     }
-}
+  }
 
-source_set("unit_tests") {
+  source_set("unit_tests") {
     testonly = true
     sources = [ "media_drm_storage_impl_unittest.cc" ]
     deps = [
@@ -45,4 +48,5 @@ source_set("unit_tests") {
       "//media/mojo/services",
       "//testing/gtest",
     ]
+  }
 }