Retire DeviceMemoryHeader flag

Device-Memory Client Hints Header shipped in M61.
This client header exposes the device Memory to web applications.
https://chromium-review.googlesource.com/579795
https://www.chromestatus.com/feature/5741299856572416

BUG=710702

Change-Id: I33213321cb42ce72e6b5dc9594605d63d12f1a01
Reviewed-on: https://chromium-review.googlesource.com/1165046Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Eric Willigers <ericwilligers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581368}

third_party/blink/renderer/platform/loader/fetch/client_hints_preferences.cc

@@ -30,8 +30,7 @@ void ParseAcceptChHeader(const String& header_value,
 
   enabled_hints.SetIsEnabled(
       mojom::WebClientHintsType::kDeviceMemory,
-      enabled_hints.IsEnabled(mojom::WebClientHintsType::kDeviceMemory) &&
-          RuntimeEnabledFeatures::DeviceMemoryHeaderEnabled());
+      enabled_hints.IsEnabled(mojom::WebClientHintsType::kDeviceMemory));
 
   enabled_hints.SetIsEnabled(
       mojom::WebClientHintsType::kRtt,

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -376,10 +376,6 @@
       name: "DesktopCaptureDisableLocalEchoControl",
       status: "experimental",
     },
-    {
-      name: "DeviceMemoryHeader",
-      status: "stable",
-    },
     {
       name: "DisableHardwareNoiseSuppression",
       origin_trial_feature_name: "DisableHardwareNoiseSuppression",