Revert "Disable enable-floating-virtual-keyboard flag by default."

This reverts commit 01873a4e.

Reason for revert: This CL was cherry-picked to M-67 release branch. Let's revert this on master.

Original change's description:
> Disable enable-floating-virtual-keyboard flag by default.
> 
> This feature is not ready for M-67 stable.
> This CL will be reverted shortly after merging it to M-67 branch.
> 
> Bug: 771466, 836109
> Test: Manual testing on the device. Confirmed that the feature is disabled by default.
> Change-Id: I295319b4bdd6ae2cec18b5690308df96cff669dd
> Reviewed-on: https://chromium-review.googlesource.com/1025352
> Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
> Commit-Queue: Yuichiro Hanada <yhanada@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#553395}

TBR=sadrul@chromium.org,yhanada@chromium.org

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

Bug: 771466, 836109
Change-Id: Ia0b300bd06d6ed0e25d8cb15dfe48013e611b592
Reviewed-on: https://chromium-review.googlesource.com/1029810Reviewed-by: Yuichiro Hanada <yhanada@chromium.org>
Commit-Queue: Yuichiro Hanada <yhanada@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553957}

ui/base/ui_base_features.cc

@@ -19,7 +19,7 @@ const base::Feature kEnableEmojiContextMenu{"EnableEmojiContextMenu",
 
 // Enables the floating virtual keyboard behavior.
 const base::Feature kEnableFloatingVirtualKeyboard = {
-    "enable-floating-virtual-keyboard", base::FEATURE_DISABLED_BY_DEFAULT};
+    "enable-floating-virtual-keyboard", base::FEATURE_ENABLED_BY_DEFAULT};
 
 // Enables the full screen handwriting virtual keyboard behavior.
 const base::Feature kEnableFullscreenHandwritingVirtualKeyboard = {