Provide defines as local variables in if-expressions.

Provide defines as local variables in if-expressions.

If-expressions can now access the values of variables defined at the
command line (using -D or -E). Undefined variables default to False.
This enables many if-expressions to be simplified, e.g.:

Before: <if expr="pp_ifdef('enable_foo')">
After:  <if expr="enable_foo">

Before: <if expr="defs['foo'] == 'bar'">
After:  <if expr="foo == 'bar'">

This also improves evaluation performance by caching compiled code
objects, leading to a 3x evaluation speedup while processing
generated_resources.grd (0.35 -> 0.12 sec on my machine).

R=joi@chromium.org

Committed: https://code.google.com/p/grit-i18n/source/detail?r=150

[newt (away), 2014-02-06 04:24:12]

joi: base.py and base_unittest.py
thakis: grd_reader_unittest.py (I'm not 100% certain of the intent of
testEarlyEnoughPlatformSpecification(), but I think it's still doing its job
after my change)

thanks!

[Jói, 2014-02-07 10:53:06]

LGTM with a question.

https://codereview.chromium.org/156443002/diff/1/grit/grd_reader_unittest.py
File grit/grd_reader_unittest.py (right):

https://codereview.chromium.org/156443002/diff/1/grit/grd_reader_unittest.py#...
grit/grd_reader_unittest.py:307: </grit>''' % sys.platform
This test is intended to be a regression test for bug 23, which occurred when
the target platform wasn't set early enough. Are you sure this still tests that?

[newt (away), 2014-02-07 17:24:45]

https://codereview.chromium.org/156443002/diff/1/grit/grd_reader_unittest.py
File grit/grd_reader_unittest.py (right):

https://codereview.chromium.org/156443002/diff/1/grit/grd_reader_unittest.py#...
grit/grd_reader_unittest.py:307: </grit>''' % sys.platform
On 2014/02/07 10:53:12, Jói wrote:
> This test is intended to be a regression test for bug 23, which occurred when
> the target platform wasn't set early enough. Are you sure this still tests
that?

Yes, the test currently passes, but if I revert the change to grit/grd_reader.py
from https://codereview.chromium.org/118663003, the test fails.

This is because the value of "os" on line 302 is the target platform, so "os" ==
"linux" iff "is_linux" is True, which is what the test previously evaluated.

[newt (away), 2014-02-07 17:33:41]

https://codereview.chromium.org/156443002/diff/50002/grit/format/resource_map...
File grit/format/resource_map_unittest.py (right):

https://codereview.chromium.org/156443002/diff/50002/grit/format/resource_map...
grit/format/resource_map_unittest.py:112: <if expr="True">
This test was failing since I added an assert that if-expressions must evaluate
to a boolean. Rationale: expr="chromium_build" should fail if chromium_build is
a string, e.g. "chromium" or "chrome"... in which case implicit conversion to a
boolean is probably not what the user intended.

[newt (away), 2014-02-07 21:01:17]

Committed patchset #3 manually as r150 (presubmit successful).