http2: add a few other common headers to the shared headermap cache

This change adds additional common CORS headers and two de-facto
standard and common X- headers to the shared headermap cache to cut down
on allocations from lower-casing/canonicalization.

Change-Id: I61121925b0b28414ed6ce07190155662b0444f93
Reviewed-on: https://go-review.googlesource.com/c/net/+/442176
Auto-Submit: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Damien Neil <dneil@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Damien Neil <dneil@google.com>

http2/headermap.go

@@ -27,7 +27,14 @@ func buildCommonHeaderMaps() {
 		"accept-language",
 		"accept-ranges",
 		"age",
+		"access-control-allow-credentials",
+		"access-control-allow-headers",
+		"access-control-allow-methods",
 		"access-control-allow-origin",
+		"access-control-expose-headers",
+		"access-control-max-age",
+		"access-control-request-headers",
+		"access-control-request-method",
 		"allow",
 		"authorization",
 		"cache-control",
@@ -53,6 +60,7 @@ func buildCommonHeaderMaps() {
 		"link",
 		"location",
 		"max-forwards",
+		"origin",
 		"proxy-authenticate",
 		"proxy-authorization",
 		"range",
@@ -68,6 +76,8 @@ func buildCommonHeaderMaps() {
 		"vary",
 		"via",
 		"www-authenticate",
+		"x-forwarded-for",
+		"x-forwarded-proto",
 	}
 	commonLowerHeader = make(map[string]string, len(common))
 	commonCanonHeader = make(map[string]string, len(common))