Lower qvalue of application/signed-exchange in Accept header

Before this patch, "application/signed-exchange;v=b3" in Accept header
had the same q-value as "text/html". But if SXG and non-SXG version of a
resource are available, browser should prefer non-SXG because SXG adds
overhead.

This patch decreases the q-value of "application/signed-exchange;v=b3"
accept header value to 0.9, which is in the middle of "text/html" and
"*/*".

Bug: 1003704
Change-Id: I955e16093a601bbe5177d842128ddff405808cc3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1810120Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Kunihiko Sakamoto <ksakamoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#702313}

chrome/browser/net/chrome_accept_header_browsertest.cc

@@ -44,7 +44,7 @@ IN_PROC_BROWSER_TEST_F(ChromeAcceptHeaderTest, Check) {
     // navigation code path and behaves similarly to PDF loaded inside <iframe>.
     ASSERT_EQ(
         "text/html,application/xhtml+xml,application/xml;q=0.9,image/"
-        "webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3",
+        "webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9",
         plugin_accept_header);
   } else {
     // This is for a sub-resource request.

content/browser/net/accept_header_browsertest.cc

@@ -97,13 +97,13 @@ IN_PROC_BROWSER_TEST_F(AcceptHeaderTest, Check) {
   // ResourceType::kMainFrame
   EXPECT_EQ(
       "text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,"
-      "image/apng,*/*;q=0.8,application/signed-exchange;v=b3",
+      "image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9",
       GetFor("/accept-header.html"));
 
   // ResourceType::kSubFrame
   EXPECT_EQ(
       "text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,"
-      "image/apng,*/*;q=0.8,application/signed-exchange;v=b3",
+      "image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9",
       GetFor("/iframe.html"));
 
   // ResourceType::kStylesheet

content/browser/web_package/signed_exchange_consts.h

@@ -8,7 +8,7 @@
 namespace content {
 
 constexpr char kAcceptHeaderSignedExchangeSuffix[] =
-    ",application/signed-exchange;v=b3";
+    ",application/signed-exchange;v=b3;q=0.9";
 
 enum class SignedExchangeVersion { kUnknown, kB3 };
 

third_party/blink/web_tests/http/tests/misc/xhtml-expected.txt

-Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3
+Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9
 
 PASS: The browser asks for XHTML.