Turn on TargetedStyleRecalc with status=experimental.

Source/platform/RuntimeEnabledFeatures.in

 // http://dev.chromium.org/blink/runtime-enabled-features
 //
 // This list is used to generate RuntimeEnabledFeatures.h/cpp which contains
 // a class that stores static enablers for all experimental features.
 //
 // Each feature can be assigned a "status":
 // status=stable -> Enable this in all Blink configurations.  We are committed to these APIs indefinitely.
 // status=experimental -> In-progress features, Web Developers might play with, but are not on by default in stable.
 // status=test -> Enabled in ContentShell for testing, otherwise off.
 // Features without a status are not enabled anywhere by default.
@@ skipping 98 matching lines @@
 SpeechInput status=stable
 SpeechSynthesis status=stable
 Srcset status=stable
 Stream status=experimental
 StyleScoped status=experimental
 SubpixelFontScaling status=experimental
 SVGPaintOrder status=stable
 
 // Enable optimizations to recalculate style on the fewest possible number of
 // elements when updating classes, ids or attributes of DOM elements.
-TargetedStyleRecalc
+TargetedStyleRecalc status=experimental
 
 // Many websites disable mouse support when touch APIs are available.  We'd
 // like to enable this always but can't until more websites fix this bug.
 // Chromium sets this conditionally (eg. based on the presence of a
 // touchscreen) in ApplyWebPreferences.
 Touch status=stable
 
 TouchIconLoading
 UserSelectAll status=experimental
 VideoTrack depends_on=Media, status=stable
 WebAnimationsSVG
 WebAnimationsAPI status=experimental
 WebAudio condition=WEB_AUDIO, status=stable
 WebGLDraftExtensions status=experimental
 WebMIDI status=test
 WebKitMediaSource status=stable
 WebVTTRegions depends_on=VideoTrack, status=experimental
 WOFF2 status=experimental
 XSLT status=stable