WebCrypto: Add ECDSA algorithm (Blink side).

Spec: https://dvcs.w3.org/hg/webcrypto-api/raw-file/tip/spec/Overview.html#ecdsa

This changelist
 * Defines a new key type for EC keys
 * Implements structured clone for EC keys
 * Adds algorithm parameter parsing for WebCrypto's EcdsaParams, EcKeyGenParams and EcKeyImportParams

This change does not include any LayoutTests, as the tests depend on the Chromium side landing first.

The LayoutTests are part of: https://codereview.chromium.org/707753002/ and will be landed last.

BUG=399094

Review URL: https://codereview.chromium.org/707743002

git-svn-id: svn://svn.chromium.org/blink/trunk@185319 bbb929c8-8fbe-4397-9dbb-9b2b20218538