Fixing/disabling Windows x64 warnings

Disabled MSVC #4267 warnings in common.gypi to enable x64 builds
for Windows.
Fixed MSVC #4267 warnings in test/testsupport.
Added third_party/directxsdk to .gitignore.

With http://review.webrtc.org/1070008 landed, this should make it possible
to build for x64 on Windows.

BUG=1348
TEST=Compiling with http://review.webrtc.org/1070008 applied:
set GYP_DEFINES="target_arch=x64"
set GYP_GENERATORS=ninja
gclient sync
ninja -C out\Debug_x64

Review URL: https://webrtc-codereview.appspot.com/1060008

git-svn-id: http://webrtc.googlecode.com/svn/trunk/webrtc@3464 4adac7df-926f-26a2-2b94-8c16560cd09d
diff --git a/common_video/common_video.gyp b/common_video/common_video.gyp
index 36e2ee5..05ddd9d 100644
--- a/common_video/common_video.gyp
+++ b/common_video/common_video.gyp
@@ -89,6 +89,10 @@
             'libyuv/scaler_unittest.cc',
             'plane_unittest.cc',
           ],
+          # Disable warnings to enable Win64 build, issue 1323.
+          'msvs_disabled_warnings': [
+            4267,  # size_t to int truncation.
+          ],
         },
       ],  # targets
     }],  # include_tests