html/template: use named consts instead of their values

Use defined named constants instead of 0 literal in comparisons.

Found using https://go-critic.github.io/overview.html#namedConst-ref

Change-Id: Ic075cece248f6e51db0b3d9d9eaba7d6409c9eef
Reviewed-on: https://go-review.googlesource.com/123376
Run-TryBot: Iskander Sharipov <iskander.sharipov@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/html/template/context.go

@@ -48,19 +48,19 @@ func (c context) mangle(templateName string) string {
 		return templateName
 	}
 	s := templateName + "$htmltemplate_" + c.state.String()
-	if c.delim != 0 {
+	if c.delim != delimNone {
 		s += "_" + c.delim.String()
 	}
-	if c.urlPart != 0 {
+	if c.urlPart != urlPartNone {
 		s += "_" + c.urlPart.String()
 	}
-	if c.jsCtx != 0 {
+	if c.jsCtx != jsCtxRegexp {
 		s += "_" + c.jsCtx.String()
 	}
-	if c.attr != 0 {
+	if c.attr != attrNone {
 		s += "_" + c.attr.String()
 	}
-	if c.element != 0 {
+	if c.element != elementNone {
 		s += "_" + c.element.String()
 	}
 	return s