Drop FilterEffectBuilder::m_referenceFilters

The job of this Vector used to be to keep a reference to the nested
"reference" filters, to avoid them being destroyed. The entire filter
graph/chain is now on the GC heap, and traced via
FilterEffectBuilder::m_lastEffect, so this additional reference is of no
use.

Review-Url: https://codereview.chromium.org/2043013002
Cr-Commit-Position: refs/heads/master@{#398341}
2 files changed