Mention ECDSA support in enterprise.platformKeys

ECDSA support in chrome.enterprise.platformKeys support was introduced
in this CL:1967468 and this CL mentions its usage in the API
documentation.

Bug: chromium:1058036
Change-Id: Ie9c57f1fe7024a47d350fd6fda5e5e8a51671118
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2087618Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Commit-Queue: Omar Morsi <omorsi@google.com>
Cr-Commit-Position: refs/heads/master@{#748773}

chrome/common/extensions/api/enterprise_platform_keys.idl

@@ -22,7 +22,8 @@ namespace enterprise.platformKeys {
     // interface. The cryptographic operations, including key generation, are
     // hardware-backed.
     // <p>Only non-extractable RSASSA-PKCS1-V1_5 keys with
-    // <code>modulusLength</code> up to 2048 can be generated. Each key can be
+    // <code>modulusLength</code> up to 2048 and ECDSA with
+    // <code>namedCurve</code> P-256 can be generated. Each key can be
     // used for signing data at most once.</p>
     // <p>Keys generated on a specific <code>Token</code> cannot be used with
     // any other Tokens, nor can they be used with