Fix platform if defined build error on Linux.

Using #if instead of #if defined() was giving an error when
ANGLE_PLATFORM_WINDOWS was undefined. Instead of defining it as
zero for all platforms, use #if defined() to ensure our legacy
usage of #ifdef doesn't get messed up by #define <value> 0.

BUG=angle:825

Change-Id: Ibad51c45c4337a30feb68561b6cf54ed5dc05d9d
Reviewed-on: https://chromium-review.googlesource.com/228270
Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>