[Device-Memory] Ship Device-Memory Client Hints Header

The feature is described in https://github.com/WICG/device-memory
and the intent to ship is here:
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/8qlTjzRY9Mc

Bug: chromium:710702,chromium:718622
Change-Id: If738affc6e171c60ad4b4540c2fec19959a1561a
Reviewed-on: https://chromium-review.googlesource.com/579795Reviewed-by: Shubhie Panicker <panicker@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Fadi Meawad <fmeawad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488463}

third_party/WebKit/Source/platform/RuntimeEnabledFeatures.json5

@@ -364,7 +364,7 @@
     },
     {
       name: "DeviceMemoryHeader",
-      status: "experimental",
+      status: "stable",
     },
     {
       name: "DisplayList2dCanvas",