Only webgl requires initialization of local variables,others don't.
Extra initialization in spirv shader may affect performance.

Bug: angleproject:4952
Change-Id: I004761779067748d43c1ea2630794491f1389492
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2359608
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tobin Ehlis <tobine@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>