Reland "Split the cros_boards gclient var into two for fetching QEMU images."

This is a reland of 8f372d5f

Reason for reland:
- All hand-rolled DEPS-consuming apps have been updated.
- No changes in this CL between original & reland.

Original change's description:
> Split the cros_boards gclient var into two for fetching QEMU images.
>
> This will help with the TODO at:
> https://source.chromium.org/chromium/chromium/src/+/master:build/config/chromeos/rules.gni;drc=ded9eb5153bb13b3f8821a6fa6fac6b3f0f590fd;l=22
>
> It will also remove the need for hard-coding the names of all these
> QEMU-compatible boards.
>
> Bug: 937821
> Change-Id: Iab180fc012bd694b4908ba0654da93d42e2274fa
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2265498
> Commit-Queue: Ben Pastene <bpastene@chromium.org>
> Reviewed-by: Erik Chen <erikchen@chromium.org>
> Reviewed-by: Dirk Pranke <dpranke@google.com>
> Cr-Commit-Position: refs/heads/master@{#786005}

Bug: 937821, 1103538
Change-Id: I6124b41f5032d9daffd4b9747ec3a974b19c6db4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2299469Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#788734}

DEPS

@@ -43,6 +43,8 @@ gclient_gn_args = [
   'checkout_nacl',
   'checkout_oculus_sdk',
   'checkout_openxr',
+  'cros_boards',
+  'cros_boards_with_qemu_images',
   'mac_xcode_version',
 ]
 
@@ -154,11 +156,14 @@ vars = {
 
   # Default to the empty board. Desktop Chrome OS builds don't need cros SDK
   # dependencies. Other Chrome OS builds should always define this explicitly.
-  'cros_boards': '',
+  'cros_boards': Str(''),
+  'cros_boards_with_qemu_images': Str(''),
   # Building for CrOS is only supported on linux currently.
   'checkout_simplechrome': '(checkout_chromeos and host_os == "linux") and ("{cros_boards}" != "")',
   # Surround the board var in quotes so gclient doesn't try parsing the string
   # as an expression.
+  # TODO(crbug.com/937821): Replace uses of this var with
+  # 'cros_boards_with_qemu_images' above.
   'cros_download_vm': '(("{cros_boards}" == "amd64-generic") or ("{cros_boards}" == "betty")) or ("{cros_boards}" == "betty-pi-arc")',
   # Should we build and test for public (ie: full) CrOS images, or private
   # (ie: release) images.

build/args/chromeos/README.md

@@ -3,9 +3,12 @@ This directory is used to store GN arg mapping for Chrome OS boards.
 Files in this directory are populated by running `gclient sync` with specific
 arguments set in the .gclient file. Specifically:
 * The file must have a top-level variable set: `target_os = ["chromeos"]`
-* The `"custom_vars"` parameter of the chromium/src.git solution must include the
-  parameter: `"cros_boards": "{BOARD_NAMES}"` where `{BOARD_NAMES}` is a
+* The `"custom_vars"` parameter of the chromium/src.git solution must include
+  the parameter: `"cros_boards": "{BOARD_NAMES}"` where `{BOARD_NAMES}` is a
   colon-separated list of boards you'd like to checkout.
+* If you'd like to a checkout a QEMU-bootable image for a given board, include
+  it in the `cros_boards_with_qemu_images` var rather than the `cros_boards`
+  var.
 
 A typical .gclient file is a sibling of the src/ directory, and might look like
 this:
@@ -18,7 +21,10 @@ solutions = [
     "custom_deps": {},
     "custom_vars" : {
         "checkout_src_internal": True,
-        "cros_boards": "eve",
+        "cros_boards": "eve:kevin",
+        # If a QEMU-bootable image is desired for any board, move it from
+        # the previous var to the following:
+        "cros_boards_with_qemu_images": "amd64-generic",
     },
   },
 ]