Refactor GrGLUniformManager::UniformHandle to initialize itself by default

Refactor GrGLUniformManager::UniformHandle to initialize itself to
"invalid" state by default. This simplifies the effect
constructors. In the future, it should also help catch potential
uninitialized uniform variable usage.

Remove unneeded explicit uniform handle validity assertions before the
handle usage. The assertion will always be made when handle is
converted to index.

BUG=skia:1492
R=bsalomon@google.com

Author: kkinnunen@nvidia.com

Review URL: https://chromiumcodereview.appspot.com/22340010

git-svn-id: http://skia.googlecode.com/svn/trunk@10713 2bbb7eff-a529-9590-31e7-b0007b416f81
21 files changed