DrCov: update genhtml to handle Windows path

Commit log for first patchset:
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DrCov: update genhtml to handle Windows path

- convert \ to / in Windows path
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[bruening, 2015-11-23 15:27:51]

If the reason is explained (just for Cygwin perl?) and it doesn't hurt native
Windows perl then LGTM

https://codereview.appspot.com/274530043/diff/1/third_party/lcov/genhtml
File third_party/lcov/genhtml (right):

https://codereview.appspot.com/274530043/diff/1/third_party/lcov/genhtml#newc...
third_party/lcov/genhtml:1556: $filename =~ tr#\\#/#;
Please explain the situation that this is addressing: is it processing a Windows
cov file on Linux?  If so, it doesn't seem like this is sufficient.  If the
script is being run on Windows, which perl type?  Native perl should handle
backslash just fine.  Is this only for Cygwin perl?  Does native perl also
handle forward slashes (I would think so but worth a quick test)?  What about
MSys perl?  Will MSys perl fail due to the drive letter?  Please explain here
and in the commit log.

[zhaoqin, 2015-11-23 21:47:00]

https://codereview.appspot.com/274530043/diff/1/third_party/lcov/genhtml
File third_party/lcov/genhtml (right):

https://codereview.appspot.com/274530043/diff/1/third_party/lcov/genhtml#newc...
third_party/lcov/genhtml:1556: $filename =~ tr#\\#/#;
On 2015/11/23 15:27:51, bruening wrote:
> Please explain the situation that this is addressing: is it processing a
Windows
> cov file on Linux?  If so, it doesn't seem like this is sufficient.  If the
> script is being run on Windows, which perl type?  Native perl should handle
> backslash just fine.  Is this only for Cygwin perl?  Does native perl also
> handle forward slashes (I would think so but worth a quick test)?  What about
> MSys perl?  Will MSys perl fail due to the drive letter?  Please explain here
> and in the commit log.

It is processing Windows cov file in cygwin, and must use cygwin's per.
Quick test shows Native perl can handle both backslash and forward slash:
Found common filename prefix "d:/src/pdfium"
Writing .css and .png files.

But there are several other problems of using the native perl:
- no pwd command in cmd shell: 'pwd' is not recognized as an internal or
external command,
- genhtml try to create directory using / as dirsep.
  genhtml: ERROR: cannot create directory core/include/fpdfapi!
So we would support cygwin only for now.
will update commit log.