Ship 'left' and 'right' values for 'text-underline-position'

This patch marks these new values as "stable" in
runtime_enabled_features.json5.

Intent to ship thread:
https://groups.google.com/a/chromium.org/forum/#!msg/blink-dev/gXXMY1grZ-o/b0y3ENIaCQAJ

Bug: 313888, 854091
Change-Id: I2cb4174e644d338163ad5162c7574aaee75d3abd
Reviewed-on: https://chromium-review.googlesource.com/1226485Reviewed-by: Kent Tamura <tkent@chromium.org>
Reviewed-by: Eric Willigers <ericwilligers@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#591412}

third_party/WebKit/LayoutTests/virtual/stable/webexposed/nonstable-css-properties-expected.txt

@@ -64,20 +64,20 @@ el.style.getPropertyValue('text-justify') is
 getComputedStyle(el).getPropertyValue('text-justify') is 
 
 el.style.setProperty('text-underline-position', 'left')
-el.style.getPropertyValue('text-underline-position') is 
-getComputedStyle(el).getPropertyValue('text-underline-position') is auto
+el.style.getPropertyValue('text-underline-position') is left
+getComputedStyle(el).getPropertyValue('text-underline-position') is left
 
 el.style.setProperty('text-underline-position', 'right')
-el.style.getPropertyValue('text-underline-position') is 
-getComputedStyle(el).getPropertyValue('text-underline-position') is auto
+el.style.getPropertyValue('text-underline-position') is right
+getComputedStyle(el).getPropertyValue('text-underline-position') is right
 
 el.style.setProperty('text-underline-position', 'under left')
-el.style.getPropertyValue('text-underline-position') is 
-getComputedStyle(el).getPropertyValue('text-underline-position') is auto
+el.style.getPropertyValue('text-underline-position') is under left
+getComputedStyle(el).getPropertyValue('text-underline-position') is under left
 
 el.style.setProperty('text-underline-position', 'right under')
-el.style.getPropertyValue('text-underline-position') is 
-getComputedStyle(el).getPropertyValue('text-underline-position') is auto
+el.style.getPropertyValue('text-underline-position') is under right
+getComputedStyle(el).getPropertyValue('text-underline-position') is under right
 
 el.style.setProperty('clear', 'inline-start')
 el.style.getPropertyValue('clear') is 

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -1208,7 +1208,7 @@
     },
     {
       name: "TextUnderlinePositionLeftRight",
-      status: "experimental",
+      status: "stable",
     },
     {
       name: "TimerThrottlingForBackgroundTabs",