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@3464 4adac7df-926f-26a2-2b94-8c16560cd09d
diff --git a/.gitignore b/.gitignore
index 42bf048..d5d64cd 100644
--- a/.gitignore
+++ b/.gitignore
@@ -42,6 +42,7 @@
 /testing
 /third_party/asan
 /third_party/cygwin
+/third_party/directxsdk
 /third_party/expat
 /third_party/gaeunit
 /third_party/gold