H264 Decoder: move cut off logic from Construct to ModifyRefPicList().

H264 Decoder: move cut off logic from Construct to ModifyRefPicList().

The ConstructRefPicListsP/B() functions are supposed to be slice agnostic.

The cut off logic is based on per-slice num_ref_idx_lX_active_minus1 values
in the slice header, and therefore it should belong to the ModifyRefPicList()
function.

BUG=479953
TEST=verify no corruption on xfnity, trybot, vda unit test

Review URL: https://codereview.chromium.org/1318283007

Cr-Commit-Position: refs/heads/master@{#346947}
(cherry picked from commit 5e19b061b92dd718401660d1899fd56bcd8637f9)

TBR=posciak@chromium.org
NOTRY=true
NOPRESUBMIT=true

Review URL: https://codereview.chromium.org/1323143006

Cr-Commit-Position: refs/branch-heads/2490@{#140}
Cr-Branched-From: 7790a3535f2a81a03685eca31a32cf69ae0c114f-refs/heads/master@{#344925}
(cherry picked from commit f7db687df527e95d3c94dd59597d99f6d1bb10d0)

[hshi1, 2015-09-22 01:07:34]

The CQ bit was checked by hshi@chromium.org

[commit-bot: I haz the power, 2015-09-22 01:07:51]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1354993003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1354993003/1

[commit-bot: I haz the power, 2015-09-22 01:09:31]

Committed patchset #1 (id:1)