jquery.client: Include platform in the profile cache key

When different tests run a similar userAgent string but with
a different platform, it returns an inappropiate cache object

e.g.
 userAgent: Firefox
 platform: Linux x86_64
 -> { platform: 'linux' }
 userAgent: Firefox
 platform: MacIntel
 -> { platform: 'linux' }
 (instead of platform: 'mac' )

Change-Id: Iaf2a5711ace82cbd2b68486e9680c7a6bcb7f9da