[gn] Remove an assert that's hard to adhere.

We don't explicitly switch off v8_use_external_startup_data
when not using a snapshot. Therefore we also shouldn't assert
this.

BUG=chromium:474921
NOTRY=true

Review-Url: https://codereview.chromium.org/2127633002
Cr-Commit-Position: refs/heads/master@{#37542}

BUILD.gn

@@ -668,7 +668,7 @@ source_set("v8_maybe_snapshot") {
       ":v8_snapshot",
     ]
   } else {
-    assert(!v8_use_external_startup_data)
+    # Ignore v8_use_external_startup_data setting if no snapshot is used.
     public_deps = [
       ":v8_nosnapshot",
     ]