Reland "IDL: Fix invalid extended attributes"

This is a reland of 5c872c1a

Original change's description:
> IDL: Fix invalid extended attributes
> 
> An invliad extended attribute [Enabled] was used in IDLs of WebRTC,
> and some APIs are visible regardless a runtime enabled feature.
> This CL fixes the IDL description and hide them behind a flag.
> 
> 
> Bug: 999690, 986069
> Change-Id: I1214c717706f52dacdc0648aa8d8ddeb951ea888
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1781923
> Reviewed-by: Harald Alvestrand <hta@chromium.org>
> Reviewed-by: Kentaro Hara <haraken@chromium.org>
> Reviewed-by: Guido Urdaneta <guidou@chromium.org>
> Reviewed-by: Florent Castelli <orphis@chromium.org>
> Commit-Queue: Hitoshi Yoshida <peria@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#693038}

Bug: 999690, 986069
Change-Id: I9d225027c377935ca70948359887096a701275c5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1792036Reviewed-by: Hitoshi Yoshida <peria@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Hitoshi Yoshida <peria@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694657}

third_party/blink/renderer/modules/peerconnection/rtc_rtp_codec_capability.idl

@@ -8,5 +8,5 @@ dictionary RTCRtpCodecCapability {
     required unsigned long  clockRate;
              unsigned short channels;
              DOMString      sdpFmtpLine;
-    [Enabled=RTCSvcScalabilityMode] sequence<DOMString> scalabilityModes;
+    [RuntimeEnabled=RTCSvcScalabilityMode] sequence<DOMString> scalabilityModes;
 };

third_party/blink/renderer/modules/peerconnection/rtc_rtp_encoding_parameters.idl

@@ -21,5 +21,5 @@ dictionary RTCRtpEncodingParameters : RTCRtpCodingParameters {
     unsigned long   maxBitrate;
     //TODO(orphis): Missing maxFramerate. https://crbug.com/857047
     double          scaleResolutionDownBy;
-    [Enabled=RTCSvcScalabilityMode] DOMString scalabilityMode;
+    [RuntimeEnabled=RTCSvcScalabilityMode] DOMString scalabilityMode;
 };

third_party/blink/web_tests/external/wpt/webrtc-svc/RTCRtpParameters-scalability-expected.txt

+This is a testharness.js-based test.
+FAIL Setting scalabilityMode to a legal value should be accepted assert_equals: expected (string) "L1T3" but got (undefined) undefined
+FAIL Sender capabilities should include at least some scalability modes assert_true: expected true got false
+Harness: the test ran to completion.
+