Only make timer queries' results available when control returns to browser.

This implements the specified semantics for WebGL's
EXT_disjoint_timer_query extension. Once this is reviewed, the same
semantics will be implemented for the WebGL 2.0 context's query objects.

Tested with conformance/extensions/ext-disjoint-timer-query.html from
https://github.com/KhronosGroup/WebGL (with the command line argument
--enable-webgl-draft-extensions), which has been revised to test this
behavior. Test is available at
https://www.khronos.org/registry/webgl/sdk/tests/conformance/extensions/ext-disjoint-timer-query.html .

BUG=345227

Review URL: https://codereview.chromium.org/1385793002

Cr-Commit-Position: refs/heads/master@{#352506}
4 files changed