libchrome: Fix soong plugin.

"generator" in ninja is special variable, and its usage conflicts.
This renames the variable name to mojomGenerator to avoid unexpected
breakage of incremental builds.

Bug: 121389487
Test: Built once. Edit mojom_header_generator rule, and rebuilt.
      Didn't regenerate without this CL, but regenerated with this.

Change-Id: I9f57e86fc98c2a3c331954dfdd44a5dcd521cc74