[Image Capture] Add exposureTime constraint. This CL adds exposureTime to ImageCapture API. Spec discussion: https://github.com/w3c/mediacapture-image/issues/199 This was added to the spec in https://github.com/w3c/mediacapture-image/pull/200 Layout tests and mock tests are updated to support the same. Support for Android is added. TEST= run the demo in https://codepen.io/rijuB/pen/eKwLXB use slider to change exposureTime. BUG=823316 Intent to Implement and Ship discussions: https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/ls3wQSoHOUY Change-Id: Ic3a546d734c02df31cd8ca08ece01f006ae8d906 Reviewed-on: https://chromium-review.googlesource.com/1136439 Reviewed-by: Kinuko Yasuda <kinuko@chromium.org> Reviewed-by: Miguel Casas <mcasas@chromium.org> Reviewed-by: Guido Urdaneta <guidou@chromium.org> Commit-Queue: Rijubrata Bhaumik <rijubrata.bhaumik@intel.com> Cr-Commit-Position: refs/heads/master@{#591946} 
diff --git a/mediacapture-image/MediaStreamTrack-getCapabilities.html b/mediacapture-image/MediaStreamTrack-getCapabilities.html index eceaf5b..9e34f23 100644 --- a/mediacapture-image/MediaStreamTrack-getCapabilities.html +++ b/mediacapture-image/MediaStreamTrack-getCapabilities.html 
@@ -78,6 +78,15 @@  assert_equals(capabilities.exposureCompensation.step,  mockCapabilities.exposureCompensation.step);   + assert_true(capabilities.exposureTime instanceof + MediaSettingsRange); + assert_equals(capabilities.exposureTime.max, + mockCapabilities.exposureTime.max); + assert_equals(capabilities.exposureTime.min, + mockCapabilities.exposureTime.min); + assert_equals(capabilities.exposureTime.step, + mockCapabilities.exposureTime.step); +  assert_true(capabilities.colorTemperature instanceof  MediaSettingsRange);  assert_equals(capabilities.colorTemperature.max,