Set arm_tune to empty string if mtune flag is not set by toolchain

Since the default arm_tune option applied by chromium is not
recognized by our gcc toolchain set arm_tune to empty string
to prevent chromium from setting a default. Our toolchains
set the arm version which already implies the corresponding
mtune option anyway.

Change-Id: I0291bd4f0e5706597d37f914e4eeefbd75397bbf
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>

src/core/gyp_run.pro

@@ -52,7 +52,7 @@ cross_compile {
         !isEmpty(MARCH): GYP_ARGS += "-D arm_arch=\"$$MARCH\""
 
         MTUNE = $$extractCFlag("-mtune=.*")
-        !isEmpty(MTUNE): GYP_ARGS += "-D arm_tune=\"$$MTUNE\""
+        GYP_ARGS += "-D arm_tune=\"$$MTUNE\""
 
         MFLOAT = $$extractCFlag("-mfloat-abi=.*")
         !isEmpty(MFLOAT): GYP_ARGS += "-D arm_float_abi=\"$$MFLOAT\""