Enable ClangTidy check llvm-namespace-comment.

This fixes a large number of SkSL namespaces which were labeled as if
they were anonymous, and also a handful of other mislabeled namespaces.
Missing namespace-end comments have been added throughout.
A number of diffs are just indentation-related (adjusting 1- or 3-
space indents to 2-space).

Change-Id: I6c62052a0d3aea4ae12ca07e0c2a8587b2fce4ec
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/308503
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
diff --git a/gm/polygonoffset.cpp b/gm/polygonoffset.cpp
index 99ce47f..fe703a1 100644
--- a/gm/polygonoffset.cpp
+++ b/gm/polygonoffset.cpp
@@ -413,7 +413,7 @@
 };
 static_assert(SK_ARRAY_COUNT(gSimpleSizes) == SK_ARRAY_COUNT(gSimplePoints), "array_mismatch");
 
-}
+}  // namespace PolygonOffsetData
 
 namespace skiagm {
 
@@ -623,4 +623,4 @@
 
 DEF_GM(return new PolygonOffsetGM(true);)
 DEF_GM(return new PolygonOffsetGM(false);)
-}
+}  // namespace skiagm