Add an angle::Format for each ES2 vertex data format.  Add function
GetVertexFormatID() to get the angle::Format for a vertex attribute.
These will be used later to support vertex formats in Vulkan (by mapping
angle::Format to Vulkan format) and to eliminate the redundant enum
gl::VertexFormatType.
No functional change.

BUG=angleproject:2405
BUG=angleproject:2531

Change-Id: I871ae23ce9fba57d90c554376e84b03f8514f7fc
Reviewed-on: https://chromium-review.googlesource.com/1044874Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>