Revert "Enable V1app backbutton by default"

This reverts commit 304ec7f27e0a23583a04cb9d0888e328df253ac5.

Reason for revert:

The original patch enabled the back button on all Hosted App windows.
This included Desktop PWA Hosted Apps that had a manifest with
display:standalone which explicitly requests for no browser controls.

Reverting this change for now until we can disable the back button for
Desktop PWA Hosted Apps.

Original change's description:
> Enable V1app backbutton by default
> 
> Added about flag in case for troubleshooting.
> 
> BUG=749713
> TEST=manual
> 
> Change-Id: I2f74535282bb8dccca38a4d8ad6f10af88140c62
> Reviewed-on: https://chromium-review.googlesource.com/860522
> Commit-Queue: Mitsuru Oshima <[email protected]>
> Reviewed-by: Michael Wasserman <[email protected]>
> Cr-Commit-Position: refs/heads/master@{#529074}

[email protected],[email protected]

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 749713
Change-Id: I9d405002dc9061d42f4acb7575b4e7c08e8ce925
Reviewed-on: https://chromium-review.googlesource.com/874491
Reviewed-by: Giovanni Ortuño Urquidi <[email protected]>
Commit-Queue: Giovanni Ortuño Urquidi <[email protected]>
Cr-Commit-Position: refs/heads/master@{#530382}
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index d9c457cd..b0e44a81 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -1487,7 +1487,7 @@
 #endif  // ENABLE_PLUGINS
 #if defined(OS_CHROMEOS)
     {"ash-enable-night-light", flag_descriptions::kEnableNightLightName,
-     flag_descriptions::kEnableNightLightDescription, kOsCrOS,
+     flag_descriptions::kEnableNightLightDescription, kOsAll,
      SINGLE_VALUE_TYPE(ash::switches::kAshEnableNightLight)},
     {"allow-touchpad-three-finger-click",
      flag_descriptions::kAllowTouchpadThreeFingerClickName,
@@ -1545,10 +1545,6 @@
     {"show-touch-hud", flag_descriptions::kShowTouchHudName,
      flag_descriptions::kShowTouchHudDescription, kOsCrOS,
      SINGLE_VALUE_TYPE(ash::switches::kAshTouchHud)},
-    {"ash-disable-v1-app-back-button",
-     flag_descriptions::kAshDisableV1AppBackButtonName,
-     flag_descriptions::kAshDisableV1AppBackButtonDescription, kOsCrOS,
-     SINGLE_VALUE_TYPE(ash::switches::kAshDisableV1AppBackButton)},
 #endif  // OS_CHROMEOS
     {
         "disable-accelerated-video-decode",