[Chromecast] Allow Chromecast builds to use WV component headers.

Cast builds were broken by this change: crrev.com/c/957707

BUG=
Bug: b/74547326

Change-Id: I41cb56436af9f5657f9ed87d817a9129c8cdf544
Reviewed-on: https://chromium-review.googlesource.com/960742Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Commit-Queue: Stephen Lanham <slan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542970}

third_party/widevine/cdm/BUILD.gn

@@ -10,9 +10,10 @@ import("//media/cdm/library_cdm/ppapi_cdm_adapter.gni")
 import("//media/media_options.gni")
 import("//third_party/widevine/cdm/widevine.gni")
 
+# Internal Cast builds set enable_widevine=true to bring in Widevine support.
 # TODO(xhwang): Support component updated CDM on other platforms and remove this
 # assert.
-assert(!enable_widevine || is_win || is_mac,
+assert(!enable_widevine || is_win || is_mac || is_chromecast,
        "Component updated CDM only supported on Windows and Mac for now.")
 
 widevine_arch = current_cpu