Add kEmptyImmutableString. We can use this instead of ImmutableString(""). Bug: angleproject:2665 Change-Id: I8b3d5d3075838b9f2caa1627071202e48a5fdc83 Reviewed-on: https://chromium-review.googlesource.com/1108085 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Luc Ferron <lucferron@chromium.org>