Destroy frontend and backend for Textures

Same idea as for buffers, Destroy can be used to free GPU memory
associated with resources without waiting for javascript garbage
collection to occur.

Bug: dawn:46
Change-Id: Ia796b06b5228cbec4cfe8d78a500f967181d8c1f
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/5540
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Natasha Lee <natlee@microsoft.com>

src/dawn_native/opengl/TextureGL.cpp

@@ -119,7 +119,7 @@ namespace dawn_native { namespace opengl {
     // Texture
 
     Texture::Texture(Device* device, const TextureDescriptor* descriptor)
-        : Texture(device, descriptor, GenTexture()) {
+        : Texture(device, descriptor, GenTexture(), TextureState::OwnedInternal) {
         uint32_t width = GetSize().width;
         uint32_t height = GetSize().height;
         uint32_t levels = GetNumMipLevels();
@@ -150,14 +150,21 @@ namespace dawn_native { namespace opengl {
         glTexParameteri(mTarget, GL_TEXTURE_MAX_LEVEL, levels - 1);
     }
 
-    Texture::Texture(Device* device, const TextureDescriptor* descriptor, GLuint handle)
-        : TextureBase(device, descriptor), mHandle(handle) {
+    Texture::Texture(Device* device,
+                     const TextureDescriptor* descriptor,
+                     GLuint handle,
+                     TextureState state)
+        : TextureBase(device, descriptor, state), mHandle(handle) {
         mTarget = TargetForDimensionAndArrayLayers(GetDimension(), GetArrayLayers());
     }
 
     Texture::~Texture() {
-        // TODO(kainino@chromium.org): delete texture (but only when not using the native texture
-        // constructor?)
+        Destroy();
+    }
+
+    void Texture::DestroyImpl() {
+        glDeleteTextures(1, &mHandle);
+        mHandle = 0;
     }
 
     GLuint Texture::GetHandle() const {