cc: Turn ResourceProvider::ValidateResources into DCHECK-only code.

These CHECKs have helped us find the bugs we were searching for, so we
can make them DCHECKs now. I've opted to keep around DCHECKs rather
than removing it entirely, as crashing during swap instead of inside
AppendQuads is really hard to debug.

R=enne@chromium.org, piman@chromium.org
BUG=475894

Review URL: https://codereview.chromium.org/1084433003

Cr-Commit-Position: refs/heads/master@{#326622}

cc/resources/resource_provider.cc

@@ -2046,9 +2046,8 @@ GLint ResourceProvider::GetActiveTextureUnit(GLES2Interface* gl) {
 
 void ResourceProvider::ValidateResource(ResourceId id) {
   DCHECK(thread_checker_.CalledOnValidThread());
-  CHECK(id);
-  ResourceMap::iterator it = resources_.find(id);
-  CHECK(it != resources_.end());
+  DCHECK(id);
+  DCHECK(resources_.find(id) != resources_.end());
 }
 
 GLES2Interface* ResourceProvider::ContextGL() const {