disable inline ARM saturated arithmetic assembly for debug builds

When building third_party/WebKit/Source/wtf/asm/SaturatedArithmeticARM.h
in debug builds, we get this error:
cc1plus: warnings being treated as errors
 SaturatedArithmeticARM.h: In function 'int saturatedSet(int, int)':
 SaturatedArithmeticARM.h:70:43: error: asm operand 2 probably doesn't match constraints
 SaturatedArithmeticARM.h:70:43: error: asm operand 3 probably doesn't match constraints

By switching -O0 to -O1 or -O2 in the compile command, the compile
succeeds. So let's disable the inline ARM assembly unless the build
is optimized- gcc defines _OPTIMIZE_=1 in this case.
I think this is a reasonable workaround until we can find a "proper"
solution.

BUG=325746

Review URL: https://codereview.chromium.org/396603002

git-svn-id: svn://svn.chromium.org/blink/trunk@178209 bbb929c8-8fbe-4397-9dbb-9b2b20218538