Forward declare template specialization (media/filters)

In jumbo builds the compiler complain about not
knowing about the template specialization of the
function OnStartOfCodedFrameGroup, that is defined
in source_buffer_stream.cc. To solve this issue
a fordward declaration is done before usage.

Bug: 867350
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: If090b848bc2ae78121620aba2349eac4c63d8a65
Reviewed-on: https://chromium-review.googlesource.com/1151313
Commit-Queue: Chrome Cunningham (In Paris) <chcunningham@chromium.org>
Reviewed-by: Chrome Cunningham (In Paris) <chcunningham@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578591}
2 files changed