docs: convert ios build instructions from html to markdown

docs/ios_build_instructions.md

+### Build Instructions (iOS)
+
+**Note:** Upstreaming of iOS code is still a work in progress. In particular, 
+note that **it is not currently possible to build an actual Chromium app.**
+Currently, the buildable binaries are ios\_web\_shell (a minimal wrapper around
+the web layer), and various unit tests.
+
+Prerequisites
+-------------
+
+-   A Mac with a version of OS X capable of running the latest version
+    of Xcode.
+-   The latest version of [Xcode](https://developer.apple.com/xcode/),
+    including the current iOS SDK.
+-   The current version of the JDK (required for the closure compiler).
+-   [depot\_tools](http://dev.chromium.org/developers/how-tos/install-depot-tools).
+
+Setting Up
+----------
+
+In the directory where you are going to check out the code, create a
+`chromium.gyp_env` to set the build to use iOS targets (and to use
+hybrid builds; see Building below):
+
+`echo "{ 'GYP_DEFINES': 'OS=ios','GYP_GENERATORS':
+'ninja,xcode-ninja', }" > chromium.gyp_env`
+
+If you aren't set up to sign iOS build products via a developer account,
+you should instead use:
+
+`echo "{ 'GYP_DEFINES': 'OS=ios chromium_ios_signing=0', 'GYP_GENERATORS': 'ninja,xcode-ninja', }" > chromium.gyp_env`
+
+Also, you should [install API
+keys](https://www.chromium.org/developers/how-tos/api-keys).
+
+Getting the Code
+----------------
+
+Next, [check out the
+code](https://www.chromium.org/developers/how-tos/get-the-code), with
+
+`fetch ios`
+
+Building
+--------
+
+Build the target you are interested in. The instructions above select
+the ninja/Xcode hybrid mode, which uses ninja to do the actual build,
+but provides a wrapper Xcode project that can be used to build targets
+and navigate the source. (The Xcode project just shells out to ninja to
+do the builds, so you can't actually inspect/change target-level
+settings from within Xcode; this mode avoids generating a large tree of
+Xcode projects, which leads to performance issues in Xcode). To build
+with ninja (simulator and device, respectively):
+
+`ninja -C out/Debug-iphonesimulator All`
+
+`ninja -C out/Debug-iphoneos All`
+
+To build with Xcode, open build/all.ninja.xcworkspace, and choose the
+target you want to build.
+
+You should always be able to build all:All, since targets are added
+there for iOS only when they compile.
+
+Running
+-------
+
+Any target that is built and runs on the bots (see below) should run
+successfully in a local build. As of the time of writing, this is only
+ios\_web\_shell and unit test targets—see the note at the top of this
+page. Check the bots periodically for updates; more targets (new
+components) will come on line over time.
+
+To run in the simulator from the command line, you can use `iossim`. For
+example, to run a debug build of web\_shell:
+
+`out/Debug-iphonesimulator/iossim out/Debug-iphonesimulator/ios_web_shell.app`
+
+**Converting an existing Mac checkout into an iOS checkout**
+
+If you want to convert your Mac checkout into an iOS checkout, you can
+follow the next steps:
+
+1- Add 'target\_os = [ "ios" ]' to the bottom of your chromium/.gclient
+file.
+
+2- Make sure you have the following in your chromium/chromium.gyp\_env
+file (removing the `chromium_ios_signing=0` if you want to make
+developer-signed builds):
+
+{

[sdefresne, 2016/03/01 08:47:00]

Looking at the processed file at
https://chromium.googlesource.com/chromium/src/+/master/docs/ios_build_instru...,
I think we should do the following change:

`{
    "GYP\_DEFINES": "OS=ios chromium\_ios\_signing=0",
    "GYP\_GENERATORS": "ninja,xcode-ninja",
}`

[tfarina, 2016/03/01 12:51:56]

Done.

+
+"GYP\_DEFINES" : "OS=ios chromium\_ios\_signing=0",
+
+"GYP\_GENERATORS" : "ninja,xcode-ninja",
+
+}
+
+\

[sdefresne, 2016/03/01 08:47:00]

Remove.

[tfarina, 2016/03/01 12:51:56]

Done.

+
+Then make sure you sync again to get all the new files like the
+following. At the end it will run gyp\_chromium which will regenerate
+all the build files according to the new settings.
+
+\

[sdefresne, 2016/03/01 08:47:00]

Remove.

[tfarina, 2016/03/01 12:51:56]

Done.

+
+`gclient sync`
+
+\

[sdefresne, 2016/03/01 08:47:00]

Remove.

[tfarina, 2016/03/01 12:51:56]

Done.

+
+Troubleshooting
+---------------
+
+If your build fails, check the iOS columns of [the Mac
+waterfall](http://build.chromium.org/p/chromium.mac/console) (the last
+two) to see if the bots are green. In general they should be, since
+failures on those bots will close the tree.