Make WebGL extensions get lost when context is lost.

Source/core/html/canvas/WebGLCompressedTexturePVRTC.cpp

Index: Source/core/html/canvas/WebGLCompressedTexturePVRTC.cpp
diff --git a/Source/core/html/canvas/WebGLCompressedTexturePVRTC.cpp b/Source/core/html/canvas/WebGLCompressedTexturePVRTC.cpp
index 04ee4969b1ff003bba5699a0ea54eaef5a0ef0aa..2a2ce5cea6b0377a1ab19a520314f6a3530479f8 100644
--- a/Source/core/html/canvas/WebGLCompressedTexturePVRTC.cpp
+++ b/Source/core/html/canvas/WebGLCompressedTexturePVRTC.cpp
@@ -36,16 +36,21 @@ WebGLCompressedTexturePVRTC::WebGLCompressedTexturePVRTC(WebGLRenderingContext*
     : WebGLExtension(context)
 {
     ScriptWrappable::init(this);
-    context->addCompressedTextureFormat(Extensions3D::COMPRESSED_RGB_PVRTC_4BPPV1_IMG);
-    context->addCompressedTextureFormat(Extensions3D::COMPRESSED_RGB_PVRTC_2BPPV1_IMG);
-    context->addCompressedTextureFormat(Extensions3D::COMPRESSED_RGBA_PVRTC_4BPPV1_IMG);
-    context->addCompressedTextureFormat(Extensions3D::COMPRESSED_RGBA_PVRTC_2BPPV1_IMG);
+    enable();
 }
 
 WebGLCompressedTexturePVRTC::~WebGLCompressedTexturePVRTC()
 {
 }
 
+void WebGLCompressedTexturePVRTC::enable()
+{
+    m_context->addCompressedTextureFormat(Extensions3D::COMPRESSED_RGB_PVRTC_4BPPV1_IMG);
+    m_context->addCompressedTextureFormat(Extensions3D::COMPRESSED_RGB_PVRTC_2BPPV1_IMG);
+    m_context->addCompressedTextureFormat(Extensions3D::COMPRESSED_RGBA_PVRTC_4BPPV1_IMG);
+    m_context->addCompressedTextureFormat(Extensions3D::COMPRESSED_RGBA_PVRTC_2BPPV1_IMG);
+}
+
 WebGLExtension::ExtensionName WebGLCompressedTexturePVRTC::getName() const
 {
     return WebGLCompressedTexturePVRTCName;