Remove DISABLE_X86 from build.gn

Remove DISABLE_X86 from build.gn

Fix for duplicate define
../../third_party/libyuv/include/libyuv/scale_row.h:29:9: error: 'LIBYUV_DISABLE_X86' macro redefined [-Werror,-Wmacro-redefined]
        ^

GYP version relys on headers disabling the optimization.
This CL does the same for BUILD.gn
TBR=kjellander@chromium.org
BUG=libyuv:625
Committed: https://chromium.googlesource.com/libyuv/libyuv/+/e74086bfe3165ddc8616c3ceb5d2dc50c4db1d82

[fbarchard1, 2016-07-14 19:09:59]

Description was changed from

==========
Remove DISABLE_X86 from build.gn

Fix for duplicate define
../../third_party/libyuv/include/libyuv/scale_row.h:29:9: error:
'LIBYUV_DISABLE_X86' macro redefined [-Werror,-Wmacro-redefined]
        ^

R=harryjin@google.com
BUG=libyuv:625
==========

to

==========
Remove DISABLE_X86 from build.gn

Fix for duplicate define
../../third_party/libyuv/include/libyuv/scale_row.h:29:9: error:
'LIBYUV_DISABLE_X86' macro redefined [-Werror,-Wmacro-redefined]
        ^
TBR=harryjin@google.com
BUG=libyuv:625
==========

[fbarchard1, 2016-07-14 19:11:09]

fbarchard@google.com changed reviewers:
+ kjellander@chromium.org
- harryjin@google.com

[fbarchard1, 2016-07-14 19:12:48]

Description was changed from

==========
Remove DISABLE_X86 from build.gn

Fix for duplicate define
../../third_party/libyuv/include/libyuv/scale_row.h:29:9: error:
'LIBYUV_DISABLE_X86' macro redefined [-Werror,-Wmacro-redefined]
        ^
TBR=harryjin@google.com
BUG=libyuv:625
==========

to

==========
Remove DISABLE_X86 from build.gn

Fix for duplicate define
../../third_party/libyuv/include/libyuv/scale_row.h:29:9: error:
'LIBYUV_DISABLE_X86' macro redefined [-Werror,-Wmacro-redefined]
        ^

GYP version relys on headers disabling the optimization.
This CL does the same for BUILD.gn
TBR=kjellander@chromium.org
BUG=libyuv:625
==========

[fbarchard1, 2016-07-14 19:14:25]

Description was changed from

==========
Remove DISABLE_X86 from build.gn

Fix for duplicate define
../../third_party/libyuv/include/libyuv/scale_row.h:29:9: error:
'LIBYUV_DISABLE_X86' macro redefined [-Werror,-Wmacro-redefined]
        ^

GYP version relys on headers disabling the optimization.
This CL does the same for BUILD.gn
TBR=kjellander@chromium.org
BUG=libyuv:625
==========

to

==========
Remove DISABLE_X86 from build.gn

Fix for duplicate define
../../third_party/libyuv/include/libyuv/scale_row.h:29:9: error:
'LIBYUV_DISABLE_X86' macro redefined [-Werror,-Wmacro-redefined]
        ^

GYP version relys on headers disabling the optimization.
This CL does the same for BUILD.gn
TBR=kjellander@chromium.org
BUG=libyuv:625

Committed:
https://chromium.googlesource.com/libyuv/libyuv/+/e74086bfe3165ddc8616c3ceb5d...
==========

[fbarchard1, 2016-07-14 19:14:26]

Committed patchset #3 (id:40001) manually as
e74086bfe3165ddc8616c3ceb5d2dc50c4db1d82 (presubmit successful).

[kjellander_chromium, 2016-07-24 09:41:22]

lgtm