When viewing the diff of a review request not all files need to be

viewed, for example minified files don't need to be reviewed. It is

also helpful to have a way to hide files you have already reviewed.
Now files can be collapsed and expanded as the user wishes. Also,

minified files are collapsed by default.
If a minified file is also a deleted, moved/copied, binary (I'm not 

sure if this is even a possibility), or a new empty file then it will

not be collapsed by default because there will just be a short

message displayed instead of a large, one-line diff.
Collapsing a diff does not work especially elegantly when the file only

contains whitespace changes. In this case the Hide/Show whitespace

changes button can also be used to collapse the file. So the file

can't be viewed unless both buttons allow it to be. One fix for this

would be to hide the new button whenever a file is collapsed with the

Hide/Show whitespace changes button. (This fix can be done in a

separate review request)

A few small changes were also made related to chunk highlighting.

Previously the minimum height of the highlight was 1, this was changed

to 0 because when a chunk was highlighted then its file was collapsed

there would be a 1px tall highlight left behind.
Also, previously collapsing a file with either button would leave the

highlight in the same position. Now it will be updated to match the

shifted position of the chunk whenever either button is used to

collapse or expand.

A change I made to renderers.py had caused an error in the test

test_make_context_with_chunk_index, but this has been fixed now.
I have added tests to verify that the minified property added to

diffutils is working correctly (returns true if 

depot_filename contains .min., and false otherwise).
Unit tests have also been added to verify that the new context

variable collapse_file is set properly in various situations.

A couple of the new tests 

test_make_context_with_collapse_file_using_unchanged_file and

test_make_context_with_collapse_file_using_file_with_no_chunks

may be unnecessary, because I'm not sure if a file can have

'num_changes': 0 without having 'moved_or_copied': True

or 'num_chunks': 0 without 'newfile': True
Two new unit tests have been added to verify that the new onClick

event properly updates the classes of the button icon and tbodies

related to that file.

Preferably, additional unit tests would be written to verify that

the onClick event updates only the classes of elements related

to the desired file. I don't think this will be difficult, but

right now I'm not sure how to do it without doubling the size of

the html template in diffReviewableViewTests.js and possibly

needing to adjust many of the other tests in that file.

This is the only reason the review request is still WIP