Making the contructors for shader variables inline saves us from
exporting them across DLL boundaries. This makes it easier to use
them in the translator when building the translator as a DLL.

BUG=angle:466

Change-Id: Iee0556e06dc1f9e98fe9eea6577819305de0dd0b
Reviewed-on: https://chromium-review.googlesource.com/198555Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Nicolas Capens <nicolascapens@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>