pdfium should build in chromium_code mode

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What steps will reproduce the problem?
1. Edit pdfium.gyp to contain `'chromium_code': 1,` in the variables block at 
the very top
2. re-gyp
3. Build pdfium_test in a chromium checkout.

What is the expected output? What do you see instead?

It should build successfully. Instead, I get a mountain of build warnings. See 
attachment for the current list.

I'll poke at this every now and then, but I'll need help for some of this (e.g. 
the Woverloaded-virtual warnings, which are the most scary).

Original issue reported on code.google.com by thakis@chromium.org on 29 Jul 2014 at 4:49

Attachments:

• pdfiumwarn2.txt

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Down from about 5000 lines of error output to 560. Some work remains, though.

Original comment by thakis@chromium.org on 3 Aug 2014 at 10:52

Attachments:

• pdfiumwarn5.txt

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Original comment by pal...@chromium.org on 4 Aug 2014 at 8:37

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../../third_party/pdfium/fpdfsdk/src/fpdfview.cpp:28:50: error: comparison of 
integers of different signs: 'long' and 'unsigned long' [-Werror,-Wsign-compare]
    if (!newPos.IsValid() || newPos.ValueOrDie() > m_FileAccess.m_FileLen) {
                             ~~~~~~~~~~~~~~~~~~~ ^ ~~~~~~~~~~~~~~~~~~~~~~
This is from when building pdfium target.

Original comment by tfar...@chromium.org on 29 Jan 2015 at 4:12