Use clang's integrated as

The system as is throwing error when compiling atom_api_web_contents.cc:
/tmp/xxx.s: Assembler messages:
/tmp/xxx.s:23559: Internal error!
Assertion failure in get_line_subseg at ../../gas/dwarf2dbg.c line 262.

toolchain.gypi

@@ -175,11 +175,6 @@
                   }],
                 ],
               }],
-              ['clang==1', {
-                'cflags': [
-                  '-no-integrated-as',
-                ],
-              }],
               ['arm_tune!=""', {
                 'cflags': [
                   '-mtune=<(arm_tune)',