Media Controls: Increase touch radius on timeline

The timeline currently is 2px high. This converts the surrounding
margins into padding so the surrounding area will pick up touch events
and make it easier to scrub.

BUG=784363

Change-Id: I9ff370d5e0724a8e833aebc1fb2cde309dedd316
Reviewed-on: https://chromium-review.googlesource.com/819631
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523401}

third_party/WebKit/Source/modules/media_controls/resources/modernMediaControls.css

@@ -53,7 +53,6 @@ video::-webkit-media-controls-panel {
 
   min-width: 48px;
   flex: 1;
-  padding-bottom: 12px;
 
   font-size: 12px;
   font-weight: normal;  /* Make sure that we don't inherit non-defaults. */
@@ -259,8 +258,11 @@ video::-webkit-media-controls-timeline {
   -webkit-appearance: -internal-media-control;
 
   height: 2px;
-  padding: 0;
-  margin: 5px 16px;
+  padding-top: 5px;
+  padding-left: 16px;
+  padding-right: 16px;
+  padding-bottom: 17px;
+  margin: 0;
   background: transparent;
 }
 
@@ -293,6 +295,8 @@ input[pseudo="-webkit-media-controls-timeline" i]::-internal-track-segment-backg
   position: absolute;
   width: 100%;
   top: 0;
+  height: 2px;
+  overflow: hidden;
 }
 
 input[pseudo="-webkit-media-controls-timeline" i]::-internal-track-segment-highlight-before,