[Docs] Update the Mac and iOS docs for `git status` perf improvements.

docs/mac_build_instructions.md

 # Checking out and building Chromium for Mac
 
 There are instructions for other platforms linked from the 
 [get the code](get_the_code.md) page.
 
 ## Instructions for Google Employees
 
 Are you a Google employee? See
 [go/building-chrome](https://goto.google.com/building-chrome) instead.
 
@@ skipping 213 matching lines @@
 `--NSDocumentRevisionsDebugMode YES` to the launch arguments, and the `YES`
 gets interpreted as a URL to open.
 
 If you have problems building, join us in `#chromium` on `irc.freenode.net` and
 ask there. Be sure that the
 [waterfall](https://build.chromium.org/buildbot/waterfall/) is green and the
 tree is open before checking out. This will increase your chances of success.
 
 ### Improving performance of `git status`
 
+#### Increase the vnode cache size
+
 `git status` is used frequently to determine the status of your checkout.  Due
 to the large number of files in Chromium's checkout, `git status` performance
 can be quite variable.  Increasing the system's vnode cache appears to help. By
 default, this command:
 
 ```shell
 $ sysctl -a | egrep kern\..*vnodes
 ```
 
 Outputs `kern.maxvnodes: 263168` (263168 is 257 * 1024).  To increase this
 setting:
 
 ```shell
 $ sudo sysctl kern.maxvnodes=$((512*1024))
 ```
 
 Higher values may be appropriate if you routinely move between different
 Chromium checkouts.  This setting will reset on reboot, the startup setting can
 be set in `/etc/sysctl.conf`:
 
 ```shell
 $ echo kern.maxvnodes=$((512*1024)) | sudo tee -a /etc/sysctl.conf
 ```
 
 Or edit the file directly.
 
-If `git --version` reports 2.6 or higher, the following may also improve
-performance of `git status`:
+#### Configure git to use an untracked cache
+
+If `git --version` reports 2.8 or higher, try running
+
+```shell
+$ git update-index --test-untracked-cache
+```
+
+If the output ends with `OK`, then the following may also improve performance of
+`git status`:
+
+```shell
+$ git config core.untrackedCache true
+```
+
+If `git --version` reports 2.6 or higher, but below 2.8, you can instead run
 
 ```shell
 $ git update-index --untracked-cache
 ```
 
 ### Xcode license agreement
 
 If you're getting the error
 
 > Agreeing to the Xcode/iOS license requires admin privileges, please re-run as
 > root via sudo.
 
 the Xcode license hasn't been accepted yet which (contrary to the message) any
 user can do by running:
 
 ```shell
 $ xcodebuild -license
 ```
 
 Only accepting for all users of the machine requires root:
 
 ```shell
 $ sudo xcodebuild -license
 ```