Reland "Enable "ImeMozcProto" flag by default."

This reverts commit a2188b30.

Reason for revert: It's submitted by accident. Only need revert on R4147 branch.

Original change's description:
> Revert "Enable "ImeMozcProto" flag by default."
> 
> This reverts commit 1cd61996.
> 
> Reason for revert: "Kana" mode doesn't work when enable "ImeMozcProto".
> 
> BUG=chromium:1108729
> 
> Original change's description:
> > Enable "ImeMozcProto" flag by default.
> > 
> > Change-Id: I33d949c64f1432ae6ff29ab26e9a94cb7e97a2c9
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2155763
> > Reviewed-by: Darren Shen <shend@chromium.org>
> > Commit-Queue: David Vallet <dvallet@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#760747}
> 
> TBR=dvallet@chromium.org,shend@chromium.org,googleo@chromium.org,wuyingbing@chromium.org
> 
> # Not skipping CQ checks because original CL landed > 1 day ago.
> 
> Change-Id: I78651fa9deca36864583bbb76d93465783488dac
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2319085
> Reviewed-by: Darren Shen <shend@chromium.org>
> Commit-Queue: Darren Shen <shend@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#791675}

TBR=dvallet@chromium.org,shend@chromium.org,googleo@chromium.org,wuyingbing@chromium.org

# Not skipping CQ checks because this is a reland.

Bug: chromium:1108729
Change-Id: I1e68974bfb61949d5471b6809d0631add86967f8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2319930Reviewed-by: David Vallet <dvallet@chromium.org>
Commit-Queue: David Vallet <dvallet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792074}

chromeos/constants/chromeos_features.cc

@@ -503,7 +503,7 @@ const base::Feature kVirtualKeyboardFloatingResizable{
 
 // Enable or disable MOZC IME to use protobuf as interactive message format.
 const base::Feature kImeMozcProto{"ImeMozcProto",
-                                  base::FEATURE_DISABLED_BY_DEFAULT};
+                                  base::FEATURE_ENABLED_BY_DEFAULT};
 
 ////////////////////////////////////////////////////////////////////////////////