Build changes to support other embedders other than chromium

The embedder is supposed to:
1) Set perfetto_build_with_embedder = true in its
   build_overrides/build.gni
2) Define protobuf targets and have a proto_library.gni
   that is API compatible with perfetto or chromium ones.
3) Set perfetto_protobuf_target_prefix to the GN path
   that contains the proto targets.

This change:
- Introduces a perfetto_build_with_embedder bool.
- Rename build_with_android -> perfetto_build_with_android
- Rename build_standalone -> perfetto_build_standalone

Change-Id: Ib8f4ab5be494c591e980158c1d2ea018b0f6298b
diff --git a/gn/ipc_library.gni b/gn/ipc_library.gni
index 5d0df92..064f4eb 100644
--- a/gn/ipc_library.gni
+++ b/gn/ipc_library.gni
@@ -12,9 +12,9 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-import("//build_overrides/build.gni")
+import("perfetto.gni")
 
-if (!build_with_chromium) {
+if (perfetto_build_standalone || perfetto_build_with_android) {
   import("//gn/standalone/proto_library.gni")
 } else {
   import("//third_party/protobuf/proto_library.gni")