Switch the GPU helper from "allow-unsigned-executable-memory" to "allow-jit".

Swiftshader now passes MAP_JIT when allocating its code pages, so this
entitlement can be locked down.

Bug: 985816
Change-Id: Id71cca3ed7300da7990085a19652bbba5b6c92c0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1742835Reviewed-by: Mark Mentovai <mark@chromium.org>
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#686537}

chrome/app/helper-gpu-entitlements.plist

@@ -2,7 +2,7 @@
 <!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
 <plist version="1.0">
 <dict>
-	<key>com.apple.security.cs.allow-unsigned-executable-memory</key>
+	<key>com.apple.security.cs.allow-jit</key>
 	<true/>
 </dict>
 </plist>