vdatest: Initialize weak decoder pointer and factory.

Add missing initialization of decoder weak factory.

TEST=vdatest for SlowRendering/*
BUG=604302

Review URL: https://codereview.chromium.org/1918823007

Cr-Commit-Position: refs/heads/master@{#390014}
(cherry picked from commit 8eda59c8ce1cdbfcec2ba7f2223347d400e19876)

Review URL: https://codereview.chromium.org/1930703003 .

Cr-Commit-Position: refs/branch-heads/2704@{#284}
Cr-Branched-From: 6e53600def8f60d8c632fadc70d7c1939ccea347-refs/heads/master@{#386251}
1 file changed