This CL switches the GLSL Generator to use utils::StringStream. The line
writer is converted internally as well, although it converts to
`std::ostream` for now. This caused a couple of the MSL, HLSL and GLSL
tests to generate slightly fewer decimal points in a couple tests.
Bug: tint:1686
Change-Id: I9ec8c1a5ef49679fc1c9a9aece86ab3390e103fc
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/121880
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>