Make WebGL extensions get lost when context is lost.

Source/core/html/canvas/OESTextureHalfFloatLinear.cpp

Index: Source/core/html/canvas/OESTextureHalfFloatLinear.cpp
diff --git a/Source/core/html/canvas/OESTextureHalfFloatLinear.cpp b/Source/core/html/canvas/OESTextureHalfFloatLinear.cpp
index 2b51eb4e431b50bf5af37a09395e8adc8b458b34..25aeea29d40a3375c448314ece983b7732f172d6 100644
--- a/Source/core/html/canvas/OESTextureHalfFloatLinear.cpp
+++ b/Source/core/html/canvas/OESTextureHalfFloatLinear.cpp
@@ -35,13 +35,18 @@ OESTextureHalfFloatLinear::OESTextureHalfFloatLinear(WebGLRenderingContext* cont
     : WebGLExtension(context)
 {
     ScriptWrappable::init(this);
-    context->graphicsContext3D()->getExtensions()->ensureEnabled("GL_OES_texture_half_float_linear");
+    enable();
 }
 
 OESTextureHalfFloatLinear::~OESTextureHalfFloatLinear()
 {
 }
 
+void OESTextureHalfFloatLinear::enable()
+{
+    m_context->graphicsContext3D()->getExtensions()->ensureEnabled("GL_OES_texture_half_float_linear");
+}
+
 WebGLExtension::ExtensionName OESTextureHalfFloatLinear::getName() const
 {
     return OESTextureHalfFloatLinearName;