Rename namespace dawn_native to dawn::native.

But keep a namespace alias to avoid breaking project that depend on the
previous namespace name while they get updated.

Done with through the following steps:

 - git grep -l dawn_native:: | xargs sed -i "" "s/dawn_native::/dawn::native::/g"
 - git grep -l "namespace dawn_native" | xargs sed -i "" "s/namespace dawn_native/namespace dawn::native/g"
 - git cl format
 - Manual fixups in generator/templates (and the addition of
   namespace_case in dawn_json_generator.py).
 - The addition of the namespace alias in DawnNative.h

Bug: dawn:824
Change-Id: I676cc4e3ced2e0e4bab32a0d66d7eaf9537e3f09
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/75982
Reviewed-by: Loko Kung <lokokung@google.com>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Auto-Submit: Corentin Wallez <cwallez@chromium.org>