[ios] Use a default filter for the codesigning identity

The webrtc bots don't specify a default filter and are broken since
https://crrev.com/c/2198780 landed. Provide one default identity to
fix them until the webrtc bot have been configured to set a filter.

Bug: 1085056, webrtc:11591
Change-Id: I67cdff9698cfd1de1d0f83bf7045e38508367bc9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2210249
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Eugene But <eugenebut@chromium.org>
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#770687}

build/config/ios/ios_sdk.gni

@@ -38,7 +38,7 @@ declare_args() {
   # Pattern used to select the identity to use for codesigning. If defined,
   # must be a substring of the description of exactly one of the identities by
   # `security find-identity -v -p codesigning`.
-  ios_code_signing_identity_description = ""
+  ios_code_signing_identity_description = "Apple Development"
 
   # Prefix for CFBundleIdentifier property of iOS bundles (correspond to the
   # "Organization Identifier" in Xcode). Code signing will fail if no mobile