[cr92 followup] Revert using precompiled headers with unit/browser tests targets.

[mkarolin]

Using precompiled headers is causing an intermittent build error with brave_unit_tests on Windows:

../../brave/vendor/brave_base/random_unittest.cc(92,22): error: use of undeclared identifier 'M_LN2'
  EXPECT_DOUBLE_EQ(3*M_LN2, StdExponential(0, 0.25));
                     ^

This is because M_LN2 on Windows requires defining _USE_MATH_DEFINES before including <cmath>. We also include <cmath> from other unit tests, but without the macro defined. Depending on which file gets compiled first the precompiled header for cmath may end up with or without M_LN2 being defined.

Let's revert using precompiled headers for now.