Use "angle_" prefix for all targets to disambiguate them from any
potential Chromium-side targets.

Combine compiler and preprocessor tests into angle_compiler_tests.
(Chromium's "angle_unittests" target contains both.) Deleted
preprocessor_test_main.cpp.

BUG=angleproject:833

Change-Id: I7acc5a76f49c15d0064c16d2ace256f9cc86e9e8
Reviewed-on: https://chromium-review.googlesource.com/231185Reviewed-by: Kenneth Russell <kbr@chromium.org>
Tested-by: Kenneth Russell <kbr@chromium.org>