Devtools: Add device frames to device mode

Devtools: Add device frames to device mode

Adds device frame option which is enabled by default. As devices have a fixed size, rulers are updated to only show the constrained size.

[samli, 2016-02-11 04:31:18]

Description was changed from

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Devtools: Add device frames to device mode
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to

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Devtools: Add device frames to device mode

Adds device frame option which is enabled by default. As devices have a fixed
size, rulers are updated to only show the constrained size.
==========

[samli, 2016-02-11 04:31:18]

samli@chromium.org changed reviewers:
+ dgozman@chromium.org

[samli, 2016-02-11 04:39:24]

samli@chromium.org changed reviewers:
+ paulirish@chromium.org

[samli, 2016-02-11 04:39:25]

Screenshots: (http://imgur.com/a/6CrQm)
dgozman@: PTAL at code. Images are not included since I'm not sure we have the
appropriate rights to include the ones I used here. 
paulirish@: Please look into whether or not we can use those images.

https://codereview.chromium.org/1688113002/diff/20001/third_party/WebKit/Sour...
File third_party/WebKit/Source/devtools/devtools.gypi (right):

https://codereview.chromium.org/1688113002/diff/20001/third_party/WebKit/Sour...
third_party/WebKit/Source/devtools/devtools.gypi:451:
'front_end/emulated_devices/art-iphone-5s-silver-vertical.png',
Pending update from Paul regarding images:

TODO: iphone5
TODO: horizontal images

[dgozman, 2016-02-11 18:23:34]

mmccoy@ is already working on this: https://codereview.chromium.org/1650243004/.
Could you please collaborate?