Clear caches at end of CssInliner::inlineCss() #1568
Description
They are currently cleared at the start, but left hanging around potentially occupying memory after the method completes.
The caches in CssInliner itself may not be a problem, as they are object properties which will (hopefully) be garbage-collected.
But the cache in DeclarationBlockParser is static, and should be cleared after use - see #1567.
Clearing should be done in a finally to cover the case when an exception it thrown.