DevTools: Correct Source Panel Show/Hide Toolbar buttons

Regression : Misalignment of 'Hide Navigator' and 'Hide Debugger' buttons is seen in devtools.

Flipped mask positions and removed—now unnecessary—rotation transforms around show/hide panel icon containers.

BUG=671930
R=lushnikov@chromium.org
TEST=Open Dev Tools, open Sources Pane, observe show/hide sidebar buttons
are correctly aligned. crbug.com/671930

Review-Url: https://codereview.chromium.org/2600493002
Cr-Commit-Position: refs/heads/master@{#445339}

AUTHORS

@@ -295,6 +295,7 @@ Jaeseok Yoon <yjaeseok@gmail.com>
 Jaideep Bajwa <bjaideep@ca.ibm.com>
 Jaime Soriano Pastor <jsorianopastor@gmail.com>
 Jake Helfert <jake@helfert.us>
+Jake Hendy <me@jakehendy.com>
 Jakob Weigert <jakob.j.w@googlemail.com>
 Jakub Machacek <xtreit@gmail.com>
 James Choi <jchoi42@pha.jhu.edu>

third_party/WebKit/Source/devtools/front_end/ui/Icon.js

@@ -169,10 +169,8 @@ UI.Icon.Descriptors = {
 
   'largeicon-show-left-sidebar': {x: -160, y: -72, width: 28, height: 24, spritesheet: 'largeicons', isMask: true},
   'largeicon-hide-left-sidebar': {x: -192, y: -72, width: 28, height: 24, spritesheet: 'largeicons', isMask: true},
-  'largeicon-show-right-sidebar':
-      {x: -160, y: -72, width: 28, height: 24, spritesheet: 'largeicons', isMask: true, transform: 'rotate(180deg)'},
-  'largeicon-hide-right-sidebar':
-      {x: -192, y: -72, width: 28, height: 24, spritesheet: 'largeicons', isMask: true, transform: 'rotate(180deg)'},
+  'largeicon-show-right-sidebar': {x: -192, y: -72, width: 28, height: 24, spritesheet: 'largeicons', isMask: true},
+  'largeicon-hide-right-sidebar': {x: -160, y: -72, width: 28, height: 24, spritesheet: 'largeicons', isMask: true},
   'largeicon-show-top-sidebar': {
     x: -160,
     y: -72,