[webcrypto] Add the KeyAlgorithm interface.

Source/platform/blink_platform.gypi

 {
   'variables': {
     'platform_files': [
       'AsyncFileSystemCallbacks.h',
       'CalculationValue.cpp',
       'CalculationValue.h',
       'CheckedInt.h',
       'Clock.cpp',
       'Clock.h',
       'ColorChooser.cpp',
@@ skipping 244 matching lines @@
       'exported/Platform.cpp',
       'exported/WebActiveGestureAnimation.cpp',
       'exported/WebActiveGestureAnimation.h',
       'exported/WebArrayBuffer.cpp',
       'exported/WebAudioBus.cpp',
       'exported/WebAudioDevice.cpp',
       'exported/WebBlobData.cpp',
       'exported/WebContentDecryptionModule.cpp',
       'exported/WebContentDecryptionModuleSession.cpp',
       'exported/WebCryptoAlgorithm.cpp',
+      'exported/WebCryptoKeyAlgorithm.cpp',
       'exported/WebCryptoKey.cpp',
       'exported/WebCryptoResult.cpp',
       'exported/WebCursorInfo.cpp',
       'exported/WebData.cpp',
       'exported/WebDeviceMotionData.cpp',
       'exported/WebDeviceOrientationData.cpp',
       'exported/WebFileSystemCallbacks.cpp',
       'exported/WebFilterKeyframe.cpp',
       'exported/WebFloatQuad.cpp',
       'exported/WebHTTPBody.cpp',
@@ skipping 633 matching lines @@
       'animation/TimingFunctionTestHelper.cpp',
       'animation/TimingFunctionTestHelper.h',
       'animation/TimingFunctionTestHelperTest.cpp',
       'graphics/DeferredImageDecoderTest.cpp',
       'graphics/ImageDecodingStoreTest.cpp',
       'graphics/ImageFrameGeneratorTest.cpp',
       'graphics/test/MockImageDecoder.h',
     ],
   },
 }