Enable the MacV2Sandbox feature by default.

Changes in sandbox behavior can have couple-pixel adjustments to controls
drawn with Cocoa, like scrollbars. This moves some affected layout test
baselines from mac10.12 to mac10.10 as a result. The baseline search path
for macOS 10.10 is 10.10->10.11->10.12->mac->generic. Enabling the V2
sandbox means that mac10.11 and 10.12 now behave like "mac", and now
only 10.10 has a differing result for these tests.

Bug: 749839, 689306
Change-Id: I9133a74c39fe995282daf950efe6d8b0fbcb2b4f
Reviewed-on: https://chromium-review.googlesource.com/1132398Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576088}

content/public/common/content_features.cc

@@ -646,7 +646,7 @@ const base::Feature kIOSurfaceCapturer{"IOSurfaceCapturer",
 // The V2 sandbox on MacOS removes the unsandboed warmup phase and sandboxes the
 // entire life of the process.
 const base::Feature kMacV2Sandbox{"MacV2Sandbox",
-                                  base::FEATURE_DISABLED_BY_DEFAULT};
+                                  base::FEATURE_ENABLED_BY_DEFAULT};
 #endif  // defined(OS_MACOSX)
 
 #if defined(OS_ANDROID) || defined(OS_CHROMEOS)