[core] Fix google-build-namespaces errors in header files

As reported by clang-tidy-8.

src/mbgl/algorithm/update_tile_masks.hpp

@@ -9,8 +9,6 @@
 namespace mbgl {
 namespace algorithm {
 
-namespace {
-
 template <typename T>
 bool tileNeedsMask(const std::reference_wrapper<T>& tile) { return tile.get().usedByRenderedLayers; }
 template <typename T> 
@@ -55,8 +53,6 @@ void computeTileMasks(
     mask.emplace(diffZ, ref.canonical.x - (root.x << diffZ), ref.canonical.y - (root.y << diffZ));
 }
 
-} // namespace
-
 // Updates the TileMasks for all renderable tiles. Each renderable tile has a corresponding UnwrappedTileID
 // indicating where it should be rendered on the screen. A TileMask describes all regions
 // within a renderable tile that are *not* covered by other renderable tiles.

src/mbgl/gfx/attribute.hpp

@@ -24,8 +24,6 @@
 namespace mbgl {
 namespace gfx {
 
-namespace {
-
 template <typename, std::size_t> struct AttributeDataTypeOf;
 template <> struct AttributeDataTypeOf<int8_t, 1> : std::integral_constant<AttributeDataType, AttributeDataType::Byte> {};
 template <> struct AttributeDataTypeOf<int8_t, 2> : std::integral_constant<AttributeDataType, AttributeDataType::Byte2> {};
@@ -56,8 +54,6 @@ template <> struct AttributeDataTypeOf<float, 2> : std::integral_constant<Attrib
 template <> struct AttributeDataTypeOf<float, 3> : std::integral_constant<AttributeDataType, AttributeDataType::Float3> {};
 template <> struct AttributeDataTypeOf<float, 4> : std::integral_constant<AttributeDataType, AttributeDataType::Float4> {};
 
-} // namespace
-
 template <typename T, std::size_t N>
 class AttributeType {
 public:

src/mbgl/tile/tile_loader_impl.hpp

@@ -8,13 +8,11 @@
 
 #include <cassert>
 
-namespace {
+namespace mbgl {
+
 inline std::exception_ptr getCantLoadTileError() {
     return std::make_exception_ptr(std::runtime_error("Can't load tile."));
 }
-} // namespace
-
-namespace mbgl {
 
 template <typename T>
 TileLoader<T>::TileLoader(T& tile_,