A11y image labeling: reenable feature by default.

Now that we've merged the logic to default-disable this feature in M75
(crrev.com/c/1607711), we will reenable this feature on trunk for dev / canary
users.

Change-Id: If250487c206c97cbe7eb6cece72fb5f9d3897e06
Bug: 945228
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1617056Reviewed-by: Jia Meng <jiameng@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Michael Martis <martis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661263}

content/public/common/content_features.cc

@@ -130,7 +130,7 @@ const base::Feature kDesktopCaptureChangeSource{
 // When a screen reader is detected, allow users the option of letting
 // Google provide descriptions for unlabeled images.
 const base::Feature kExperimentalAccessibilityLabels{
-    "ExperimentalAccessibilityLabels", base::FEATURE_DISABLED_BY_DEFAULT};
+    "ExperimentalAccessibilityLabels", base::FEATURE_ENABLED_BY_DEFAULT};
 
 // Throttle tasks in Blink background timer queues based on CPU budgets
 // for the background tab. Bug: https://crbug.com/639852.