Remove the build_ffmpegsumo build variable.

build_ffmpegsumo no longer serves a purpose, e.g., it's not possible to
build with media_use_ffmpeg=1 and build_ffmpegsumo=0.
This rolls third_party/ffmpeg to pick up the new build files with
build_ffmpegsumo removed.

Review URL: https://codereview.chromium.org/1234083004

Cr-Original-Commit-Position: refs/heads/master@{#339320}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 701cd4b29d05cf564d91ef2a4c536823f691ee73

common.gypi

@@ -1810,7 +1810,6 @@
         'safe_browsing%': 2,
         'enable_web_speech%': 0,
         'java_bridge%': 1,
-        'build_ffmpegsumo%': 0,
         'use_allocator%': 'none',
 
         # Disable Native Client.