dav1d: Expand is_linux to is_linux || is_chromeos.

Currently is_linux is set to true on Chrome OS build,
but it is planned to set it false. This is the preparation
to keep the current behavior for dav1d.

BUG=1131947, 1110266
TEST=Built locally.

Change-Id: I0e93a3d98f3587ea40ed704dfe87019e19b8e327
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2428265
Commit-Queue: Hidehiko Abe <hidehiko@chromium.org>
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Auto-Submit: Hidehiko Abe <hidehiko@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810471}

third_party/dav1d/BUILD.gn

@@ -14,7 +14,8 @@ enable_nasm = (current_cpu == "x86" || current_cpu == "x64") && !is_msan
 if (is_win) {
   platform_config_root = "config/win/$current_cpu"
 } else if (is_msan) {
-  assert(current_cpu == "x64" && is_linux, "Only Linux X64 MSAN is supported")
+  assert(current_cpu == "x64" && (is_linux || is_chromeos),
+         "Only Linux X64 MSAN is supported")
   platform_config_root = "config/linux-noasm/$current_cpu"
 } else {
   # Linux configuration files seem to work on Mac, so just reuse them.