docs: convert chromeos build instructions from html to markdown

docs/chromeos_build_instructions.md

+# ChromeOS Build Instructions (Chromium OS on Linux)
+
+Chromium on Chromium OS is built on a mix of code sourced from Chromium
+on Linux and Chromium on Windows. Much of the user interface code is
+shared with Chromium on Windows. As such, if you make a change to
+Chromium on Windows you may find your changes affect Chromium on
+Chromium OS. Fortunately to test the effect of your changes you don't
+have to build all of Chromium OS, you can just build Chromium for
+Chromium OS directly on Linux.
+
+First, follow the [normal Linux build
+instructions](https://chromium.googlesource.com/chromium/src/+/master/docs/linux_build_instructions.md)
+as usual to get a Chromium checkout.
+
+## Running Chromium on your local machine
+
+If you plan to test the Chromium build on your dev machine and not a
+Chromium OS device run:
+
+```shell
+export GYP_DEFINES="chromeos=1"
+gclient runhooks
+```
+
+Now, once you build, you will build with Chromium OS features turned on.
+
+### Notes
+
+When you build Chromium OS Chromium, you'll be using the TOOLKIT\_VIEWS
+front-end just like Windows, so the files you'll probably want are in
+src/ui/views and src/chrome/browser/ui/views.
+
+If chromeos=1 is specified, then toolkit\_views=0 must not be specified.
+
+The Chromium OS build requires a functioning GL so if you plan on
+testing it through Chromium Remote Desktop you might face drawing
+problems (e.g. Aura window not painting anything). Possible remedies:
+
+*   --ui-enable-software-compositing --ui-disable-threaded-compositing
+*   --use-gl=osmesa, but it's ultra slow, and you'll have to build
+    osmesa yourself.
+*   ... or just don't use Remote Desktop. :)
+
+Note the underscore in the GYP_DEFINES variable name, as people
+sometimes mistakenly write it GYPDEFINES.
+
+To more closely match the UI used on devices, you can install fonts used
+by Chrome OS, such as Roboto, on your Linux distro.
+
+To specify a logged in user:
+
+*   For first run, add the following options to the command line:
+    **--user-data-dir=/tmp/chrome --login-manager**
+*   Go through the out-of-the-box UX and sign in as
+    **username@gmail.com**
+*   For subsequent runs, add the following to the command line:
+    **--user-data-dir=/tmp/chrome --login-user=username@gmail.com**.
+*   To run in guest mode instantly, you can run add the arguments
+    **--user-data-dir=/tmp/chrome --bwsi --incognito
+    --login-user='$guest' --login-profile=user**
+
+Signing in as a specific user is useful for debugging features like sync
+that require a logged in user.
+
+### Compile Testing Chromium with the Chromium OS SDK (quick version)
+
+Note: These instructions are intended for Chromium developers trying to
+diagnose compile issues on Chromium OS, which can block changes in the
+CQ. See the [full
+documentation](http://www.chromium.org/chromium-os/how-tos-and-troubleshooting/building-chromium-browser)
+for more information about building & testing chromium for Chromium OS.
+
+To do a build of Chromium that can run on Chromium OS itself, the Chromium OS
+SDK must be used. The SDK provides all of chromium's dependencies as they are
+distributed with Chromium OS (as opposed to other distributions such as Ubuntu).
+
+To enter the SDK build environment, run the following command (replace the value
+of the `--board` flag with the name of the configuration you want to test).
+
+```shell
+cros chrome-sdk --board=amd64-generic --use-external-config
+```
+
+Once in the SDK build environment, build using the normal linux workflow (except
+for a different build directory):
+
+```shell
+gclient runhooks
+ninja -C out_amd64-generic/Release chromium_builder_tests
+```
+
+The current configurations verified by the CQ are:
+
+  Board Flag | Build Directory | CPU architecture
+  --- | --- | ---
+  amd64-generic | out_amd64-generic | 64-bit Intel
+  x86-generic | out_x86-generic | 32-bit Intel
+  daisy | out_daisy | 32-bit ARM
+
+## Running Chromium on a Chromium OS device
+
+Look at the [Chromium OS
+documentation](http://www.chromium.org/chromium-os/how-tos-and-troubleshooting/building-chromium-browser)
+for the official flow for doing this.