Fix the clearProgramCompletionQueries crash

It seems that ContextGL() might be null in the destructor of
WebGLRenderingContextBase.

Bug: 962800
Change-Id: I3aa3a5b25bf85ba9a97b2e1fc007c57cf33ec82c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1610893Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Jie A Chen <jie.a.chen@intel.com>
Cr-Commit-Position: refs/heads/master@{#661160}

third_party/blink/renderer/modules/webgl/webgl_rendering_context_base.cc

@@ -1308,8 +1308,6 @@ WebGLRenderingContextBase::~WebGLRenderingContextBase() {
   // context state.
   destruction_in_progress_ = true;
 
-  clearProgramCompletionQueries();
-
   // Now that the context and context group no longer hold on to the
   // objects they create, and now that the objects are eagerly finalized
   // rather than the context, there is very little useful work that this
@@ -1332,6 +1330,8 @@ void WebGLRenderingContextBase::DestroyContext() {
   if (!GetDrawingBuffer())
     return;
 
+  clearProgramCompletionQueries();
+
   extensions_util_.reset();
 
   base::RepeatingClosure null_closure;