docs: try to give move precise commands in Xcode-Ninja Hybrid section

docs/mac_build_instructions.md

 # Mac Build Instructions
 
 [TOC]
 
 ## Prerequisites
 
 *   A Mac running 10.9+.
 *   [Xcode](https://developer.apple.com/xcode), 5+.
 *   Install
     [gclient](http://dev.chromium.org/developers/how-tos/install-depot-tools),
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 ## Using Xcode-Ninja Hybrid
 
 While using Xcode is unsupported, GYP supports a hybrid approach of using ninja
 for building, but Xcode for editing and driving compilation. Xcode can still be
 slow, but it runs fairly well even **with indexing enabled**.
 
 With hybrid builds, compilation is still handled by ninja, and can be run by the
 command line (e.g. ninja -C out/Debug chrome) or by choosing the chrome target
 in the hybrid workspace and choosing build.
 
-To use Xcode-Ninja Hybrid, set `GYP_GENERATORS=ninja,xcode-ninja`.
+To use Xcode-Ninja Hybrid, set GYP\_GENERATORS like the following:
+
+```shell
+export GYP_GENERATORS="ninja,xcode-ninja"
+```
 
 Due to the way Xcode parses ninja output paths, it's also necessary to change
 the main gyp location to anything two directories deep. Otherwise Xcode build
-output will not be clickable. Adding
-`xcode_ninja_main_gyp=src/build/ninja/all.ninja.gyp` to your
-`GYP_GENERATOR_FLAGS` will fix this.
+output will not be clickable.
+To make this change permanent, you can edit `chromium.gyp_env` (or create it if
+it does not exists) and define GYP\_GENERATOR\_FLAGS. In general, to use hybrid
+mode, your `chromium.gyp_env` could contain the following:
 
-After generating the project files with gclient runhooks, open
-`src/build/ninja/all.ninja.xcworkspace`.
+```json
+{
+  "GYP_GENERATORS"     : "ninja,xcode-ninja",
+  "GYP_GENERATOR_FLAGS":
+      "xcode_project_version=3.2 " +
+      "xcode_ninja_main_gyp=src/build/ninja/all.ninja.gyp",
+}
+```
+
+After, generate the project files with:
+
+```shell
+gclient runhooks
+```
+
+And finally, open it:
+
+```shell
+open build/ninja/all.ninja.xcworkspace
+```
 
 You may run into a problem where http://YES is opened as a new tab every time
 you launch Chrome. To fix this, open the scheme editor for the Run scheme,
 choose the Options tab, and uncheck "Allow debugging when using document
 Versions Browser". When this option is checked, Xcode adds
 `--NSDocumentRevisionsDebugMode YES` to the launch arguments, and the `YES` gets
 interpreted as a URL to open.
 
 If you want to limit the number of targets visible, which is known to improve
 Xcode performance, add `xcode_ninja_executable_target_pattern=%target%` where
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 be set in `/etc/sysctl.conf`:
 
     echo kern.maxvnodes=$((512*1024)) | sudo tee -a /etc/sysctl.conf
 
 Or edit the file directly.
 
 If your `git --version` reports 2.6 or higher, the following may also improve
 performance of `git status`:
 
     git update-index --untracked-cache