Bug 930380 - Make sure *SOURCES only point to existing files. r=gps

gfx/skia/moz.build

gfx/tests/gtest/moz.build

ipc/chromium/moz.build

media/libtheora/lib/Makefile.in

media/libtheora/lib/moz.build

media/libvpx/moz.build

python/mozbuild/mozbuild/frontend/emitter.py

...uild/mozbuild/test/backend/data/variable_passthru/bar.cpp

...build/mozbuild/test/backend/data/variable_passthru/bar.mm

...zbuild/mozbuild/test/backend/data/variable_passthru/bar.s

...zbuild/mozbuild/test/backend/data/variable_passthru/baz.S

...zbuild/mozbuild/test/backend/data/variable_passthru/foo.S

...uild/mozbuild/test/backend/data/variable_passthru/foo.asm

...uild/mozbuild/test/backend/data/variable_passthru/foo.cpp

...build/mozbuild/test/backend/data/variable_passthru/foo.mm

...ld/mozbuild/test/backend/data/variable_passthru/moz.build

...uild/mozbuild/test/backend/data/variable_passthru/test1.c

...ld/mozbuild/test/backend/data/variable_passthru/test1.cpp

...ild/mozbuild/test/backend/data/variable_passthru/test1.mm

...uild/mozbuild/test/backend/data/variable_passthru/test2.c