Remove the default backspace event handler.

We have UseCounters showing that 0.04% of page views navigate back
via the backspace button and 0.005% of page views are after a form
interaction. The latter are often cases where the user loses
data. Years of user complaints have been enough that we think
it's the right choice to change this given the degree of pain
users feel by losing their data and because every platform
has another keyboard combination that navigates back.

We're doing this via a flag so that we can control this behavior
should there be sufficient outcry.

Review URL: https://codereview.chromium.org/1854963002

Cr-Commit-Position: refs/heads/master@{#386433}
diff --git a/third_party/WebKit/Source/core/input/EventHandler.cpp b/third_party/WebKit/Source/core/input/EventHandler.cpp
index 0de628c3..34cd374 100644
--- a/third_party/WebKit/Source/core/input/EventHandler.cpp
+++ b/third_party/WebKit/Source/core/input/EventHandler.cpp
@@ -3438,6 +3438,9 @@
 {
     ASSERT(event->type() == EventTypeNames::keydown);
 
+    if (!RuntimeEnabledFeatures::backspaceDefaultHandlerEnabled())
+        return;
+
     if (event->ctrlKey() || event->metaKey() || event->altKey())
         return;
 
diff --git a/third_party/WebKit/Source/platform/RuntimeEnabledFeatures.in b/third_party/WebKit/Source/platform/RuntimeEnabledFeatures.in
index eac0a42..b791008 100644
--- a/third_party/WebKit/Source/platform/RuntimeEnabledFeatures.in
+++ b/third_party/WebKit/Source/platform/RuntimeEnabledFeatures.in
@@ -28,6 +28,7 @@
 AudioOutputDevices status=stable
 AudioVideoTracks status=experimental
 BackgroundSync status=stable
+BackspaceDefaultHandler status=test
 CacheIgnoreSearchOption status=experimental
 Canvas2dImageChromium status=experimental
 ClientHints status=stable