DevTools: do not override access-language if it is already set.

Change-Id: I9236f428c4186a09eff5ec1212dd8e5315a4ed75
Reviewed-on: https://chromium-review.googlesource.com/1079899Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Pavel Feldman <pfeldman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#563118}

third_party/WebKit/LayoutTests/http/tests/inspector-protocol/emulation/emulation-user-agent-override-expected.txt

@@ -3,5 +3,6 @@ navigator.userAgent == Test UA
 User-Agent: Test UA
 navigator.language == en-uk
 Accept-Language: en-uk,en
+accept-language: ko
 navigator.platform == new_platform
 

third_party/WebKit/LayoutTests/http/tests/inspector-protocol/emulation/emulation-user-agent-override.js

@@ -11,6 +11,9 @@
   testRunner.log('navigator.language == ' + await session.evaluate('navigator.language'));
   await printHeader('Accept-Language');
 
+  // Do not override explicit Accept-Language header.
+  await printHeaderWithLang('Accept-Language');
+
   // Platform
   await dp.Emulation.setUserAgentOverride({userAgent: '', platform: 'new_platform'});
   testRunner.log('navigator.platform == ' + await session.evaluate('navigator.platform'));
@@ -24,5 +27,14 @@
     }
   }
 
+  async function printHeaderWithLang(name) {
+    const url = testRunner.url('resources/echo-headers.php');
+    const headers = await session.evaluateAsync(`fetch("${url}", { headers: {"accept-language": "ko"}}).then(r => r.text())`);
+    for (const header of headers.split('\n')) {
+      if (header.toLowerCase().startsWith(name.toLowerCase()))
+        testRunner.log(header);
+    }
+  }
+
   testRunner.completeTest();
 })

third_party/blink/renderer/core/inspector/inspector_emulation_agent.cc

@@ -379,7 +379,8 @@ void InspectorEmulationAgent::WillSendRequest(
   String accept_lang_override;
   state_->getString(EmulationAgentState::kAcceptLanguageOverride,
                     &accept_lang_override);
-  if (!accept_lang_override.IsEmpty()) {
+  if (!accept_lang_override.IsEmpty() &&
+      request.HttpHeaderField("Accept-Language").IsEmpty()) {
     request.SetHTTPHeaderField("Accept-Language",
                                AtomicString(accept_lang_override));
   }