Fix WebGL context attribute handling and Chrome workaround

[juj]

Fix WebGL context attribute handling for enableExtensionsByDefault and explicitSwapControl, and work around Chrome behavior that it does not return powerPreference in context attributes.

CC @kainino0x and @kenrussell (and @jdashg ) : it looks like when one creates a WebGL context with context attributes powerPreference field set in Chrome, and then later calls

var attributes = context.getContextAttributes();

on the created context, the returned attributes object does not have that powerPreference field. I don't know what the spec says if it should be there, but looks like in Firefox the powerPreference field is present.

This PR makes Emscripten return "default" as powerPreference if browser did not provide it.

Also adds unrelated quote fixes for Closure compilation.

[juj]

Looks like this gets CI green, landing so that other PRs can truck along.

[kenrussell]

Looks good. Sorry that Chrome doesn't support powerPreference yet. We have been aiming to handle it in a more automatic way but it keeps getting pushed back due to higher priority issues.

[juj]

Ohh, the missing return was because there's no powerPreference at all yet, that makes sense :) thought the issue was just that getContextAttributes() did not return whatever was added. No worries, good to hear!