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>

src/tests/DawnNativeTest.h

diff --git a/src/tests/DawnNativeTest.h b/src/tests/DawnNativeTest.h
index 91904a3..ab44e39 100644
--- a/src/tests/DawnNativeTest.h
+++ b/src/tests/DawnNativeTest.h
@@ -21,7 +21,7 @@
 #include "dawn_native/DawnNative.h"
 #include "dawn_native/ErrorData.h"
 
-namespace dawn_native {
+namespace dawn::native {
 
     // This is similar to DAWN_TRY_ASSIGN but produces a fatal GTest error if EXPR is an error.
 #define DAWN_ASSERT_AND_ASSIGN(VAR, EXPR) \
@@ -29,7 +29,7 @@
 
     void AddFatalDawnFailure(const char* expression, const ErrorData* error);
 
-}  // namespace dawn_native
+}  // namespace dawn::native
 
 class DawnNativeTest : public ::testing::Test {
   public:
@@ -42,8 +42,8 @@
     virtual WGPUDevice CreateTestDevice();
 
   protected:
-    std::unique_ptr<dawn_native::Instance> instance;
-    dawn_native::Adapter adapter;
+    std::unique_ptr<dawn::native::Instance> instance;
+    dawn::native::Adapter adapter;
     wgpu::Device device;
 
   private: