Merge 116722 - Sync with impl thread when removing references to external textures

Merge 116722 - Sync with impl thread when removing references to external textures
https://bugs.webkit.org/show_bug.cgi?id=86054

We want to ensure the client side is safe to release textures, so we
sync with the impl thread when:
- we change the texture (and we had one)
- the layer is removed from the tree (and we had a texture)
- the layer is destroyed (and we had a texture)

Reviewed by James Robinson.

Test: TextureLayerChromiumTest.

* platform/graphics/chromium/TextureLayerChromium.cpp:
(WebCore::TextureLayerChromium::~TextureLayerChromium):
(WebCore::TextureLayerChromium::setTextureId):
(WebCore::TextureLayerChromium::setLayerTreeHost):
(WebCore):
* platform/graphics/chromium/TextureLayerChromium.h:
(TextureLayerChromium):


TBR=piman@chromium.org
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=116982