Bug 1374389: Prevent compilation error due to object size being too big. r=glandium

Prevent on Windows the error:
fatal error C1128: number of sections exceeded object file format limit: compile with /bigobj

MozReview-Commit-ID: 8j0fT38gkAZ

--HG--
extra : rebase_source : fbd220a208d09023f5ff6f115404349a9d97a7f8

dom/media/moz.build

@@ -4,6 +4,8 @@
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
+FILES_PER_UNIFIED_FILE = 6
+
 with Files('*'):
     BUG_COMPONENT = ('Core', 'Audio/Video')