[NaCl SDK] Fix mysterious gtest failure in Html5FsTest::OpenForCreate

[NaCl SDK] Fix mysterious gtest failure in Html5FsTest::OpenForCreate

The failure seems to have been triggered from the recent
gtest update.  The exact revision seems to be:
https://code.google.com/p/googletest/source/detail?r=693

The issue only effects builds made with our older
gcc 4.4 toolchain. Neither asan nor valgrind report any
issues with the linux run of this test, and PNaCl build
also passes just fine so I'm assuming is a compiler bug
in the older toolchain.

The fix I found was to use "const char*" over "char []"
for local string constants.  In fact just adding a const
alone and leaving the [] syntax also fixes the issue.

Most likely there is an underlying compiler bug that
still needs to be addressed.

BUG=434821

Review URL: https://codereview.chromium.org/740023002

Cr-Commit-Position: refs/heads/master@{#304959}