Pulls out the CppFiltParser work into a worker to run in the
background and avoid the jank induced by the final
convertSymsMapToExpectedSymFormat call.
Profiling shows that after this change there is virtually no
jank when asking the geckoProfiler API for symbols.
MozReview-Commit-ID: CAKMvjLa4dl
--HG--
extra : rebase_source : de9202687ca8ffbfa5216f73fe57e97ffdbea8ef
